detection
Best Plagiarism Checkers
Compare the best plagiarism checkers as of August 2026 — Copyleaks, Scribbr, Quetext, QuillBot, Originality.ai, Grammarly, Turnitin and iThenticate, with published detection rates, database coverage, document-storage policies, pricing and which to use for a thesis, a blog or a classroom.
Quick answer: For general writing, Copyleaks is the strongest plagiarism checker available to an individual in 2026 — it caught 81.8% of plagiarised passages in Reedsy’s five-tool comparison, the highest rate in the only 2026 test we found that was not published by a vendor of one of the tools in it. For a thesis, dissertation or journal submission, use Scribbr instead: it licenses Turnitin’s technology and its web and publications database, which is far closer to what a university will match against than any web-crawling checker, and it charges a one-off $19.95 to $39.95 rather than a subscription. Quetext has the most usable free tier at 500 words per check. Turnitin cannot be bought by an individual — it is sold to institutions only — and Grammarly’s checker was the weakest of the five Reedsy tested, at 54.5%. The caveat that shapes everything below: no peer-reviewed independent benchmark of similarity-matching accuracy was published in 2026, so every accuracy figure on this page comes either from a vendor testing itself or from a single small comparison, and none has been reproduced.
This page ranks tools that find copied and closely paraphrased text by matching it against a corpus of existing documents. That is a different problem from detecting whether text was written by a language model, which we cover separately in best AI detectors. The distinction matters more in 2026 than it ever has, because AI-written text is usually original by a plagiarism checker’s standards — it will pass a similarity scan with a near-zero score while being nothing the student wrote. If your actual question is “did a machine write this?”, you are on the wrong page; start here. If your question is “did this come from somewhere else, and can I see where?”, read on.
Plagiarism checking and AI detection are not the same product
Nearly every tool on this page now sells both, in one interface, on one credit balance. They are still two different technologies answering two different questions, and conflating them is the most expensive mistake buyers make in this category.
A plagiarism checker matches strings against a corpus. It breaks your document into overlapping fragments, searches an index of web pages, journal articles, books and previously submitted papers, and returns each match with a link to the source. The output is evidence: a percentage, plus a specific document you can open and compare. Its accuracy is bounded almost entirely by what is in the index — a checker cannot find a source it has never crawled, which is why the same passage can score 5% on one tool and 60% on another.
An AI detector estimates a statistical property of the text itself. It measures how predictable the writing is against a language model’s expectations and returns a probability. There is no source document, nothing to open, nothing to compare. Independent work published in the International Journal for Educational Integrity in 2026 found Originality.ai reaching 0.69 overall accuracy and Turnitin 0.61 on an academic test set, with both performing poorly on hybrid human-plus-AI text — the form most student writing now takes.
Three practical consequences follow.
- A clean plagiarism report is not evidence that a human wrote the text. Fully AI-generated writing is novel string-by-string and typically returns a very low similarity score.
- A high similarity score is not evidence of dishonesty. Correctly quoted and cited material, reference lists, standard methods sections and common phrases all raise the number.
- The two scores fail in opposite directions. Similarity matching produces false negatives on paraphrase; AI detection produces false positives on unusual but human writing, particularly from non-native English speakers.
Where this page ranks a tool’s AI detection, it says so explicitly and defers to our AI detector rankings for the accuracy evidence.
The state of plagiarism detection: August 2026
Five things define the market as it stands today.
1. The independent evidence is thin, and thinner than for AI detection. AI detectors have been tested repeatedly in peer-reviewed venues. Similarity checkers have not. The published 2026 comparisons are Reedsy’s five-tool test, Scribbr’s own comparison and Paperpal’s own comparison — and two of those three are run by companies that sell a checker in the ranking. Each names a different winner. Treat the numbers on this page as directional.
2. Turnitin remains the institutional default and remains unavailable to individuals. Turnitin’s help centre states that it does not sell individual or single-use licences; contracts are negotiated per institution on volume, feature bundle and deployment. The practical consequence for students is that the tool their work will be judged by is not a tool they can run first, which is the entire reason the reseller market exists.
3. Detection has folded into AI-integrity suites. Copyleaks, Originality.ai, Quetext, QuillBot, Winston AI and Turnitin all now sell similarity matching and AI-writing detection as one product on one balance. Turnitin consolidated its multi-model AI ensemble into a single model in July 2026, which it says simplified the AI writing report while holding its false-positive rate below 1%.
4. The evasion layer moved from synonyms to models. Paraphrasing tools defeat naive string matching by construction, and the research now documents the model-driven version: “All That Glitters is Not Novel” shows large language models producing paraphrased versions of existing papers specifically shaped to avoid detection. Vendors have responded with semantic and cross-language matching rather than exact strings, and iThenticate 2.0’s AI writing model was updated to flag text passed through AI “bypasser” tools, reporting it under an AI-generated category rather than treating it as human.
5. The volume of AI writing in submitted work has changed what schools are buying. Turnitin reports that 14.8% of English-language submissions between October 2025 and February 2026 contained more than 80% AI-generated writing, against 3.3% between April and August 2023. That shift is why Turnitin Clarity — which records the writing process rather than scoring the finished text — entered institutional pilots in the spring 2026 semester at universities including the University of Arkansas.
Top plagiarism checkers compared (August 2026)
What independent and semi-independent testing found
Three comparisons published detection rates in 2026. They do not agree, and the reason they do not agree is worth understanding before you read any of them: a similarity checker’s score is a function of its index, so a test built from open web sources favours web-indexed tools and a test built from paywalled academic sources favours tools with journal and student-paper databases.
| Tool | Reedsy (5 tools) | Scribbr (self-published) | Paperpal (self-published) | Who ran the test |
|---|---|---|---|---|
| Copyleaks | 81.8% | Not tested | Not tested | Reedsy sells no checker |
| QuillBot | 72.7% | Not tested | 55% | Reedsy; Paperpal is a competitor |
| Scribbr | 72.7% | 88% | 70% | Reedsy tested it; Scribbr also tested itself |
| Quetext | 63.6% | Not tested | Not tested | Reedsy sells no checker |
| Grammarly | 54.5% | Not tested | Not tested | Reedsy sells no checker |
| Paperpal | Not tested | Not tested | 90% (18 of 20 samples) | Paperpal tested itself |
Reedsy is the least conflicted of the three because it does not sell a plagiarism checker, and it ranked QuillBot 8.5 out of 10 as its overall pick on the strength of citation features rather than raw detection, while recording Copyleaks as the most accurate of the five. Scribbr and Paperpal each place themselves first in their own comparisons. We have quoted all three because the disagreement is the finding.
What the vendors claim
| Tool | Claimed accuracy or coverage | Basis given by the vendor |
|---|---|---|
| Turnitin | 98%+ AI-detection accuracy above a 20% AI threshold, under 1% false positives | Internal testing, published in product guidance |
| Scribbr | 88% detection | Its own published comparison |
| Paperpal | 90% detection | Its own test of 20 samples |
| Copyleaks | No headline plagiarism figure published | Publishes a testing-methodology document |
| Quetext | No figure published | Describes DeepSearch methodology instead |
| Originality.ai | No headline plagiarism figure published | Publishes AI-detection figures separately |
| Grammarly | No figure published | Not applicable |
| Smodin | 96% AI-detection accuracy | Vendor claim, plagiarism figure not published |
None of these has been independently reproduced. Where a vendor publishes no number, we have said so rather than repeat a percentage circulating on review sites.
The best plagiarism checkers, reviewed
1. Copyleaks — best overall, and the best evidence behind it
Copyleaks is the strongest all-round choice for an individual or a small team, and the reason is the only comparatively independent number in the category: 81.8% detection in Reedsy’s 2026 five-tool test, ahead of QuillBot at 72.7% and Grammarly at 54.5%. It covers more than 100 languages, offers cross-language matching so a passage translated out of another language can still be caught, and scans source code as well as prose. It is SOC 2 and GDPR aligned, which is why it appears in institutional procurement alongside Turnitin.
The weaknesses are real. Credits do not roll over month to month on individual plans, so uneven usage wastes them. And its own free-tier performance has been questioned elsewhere: in Scribbr’s evaluation of AI detectors, Copyleaks produced no false positives but only 66% real-world accuracy on the free tier, far below the figures it advertises — a reminder that the free tier is not the product being benchmarked.
Pricing: free tier of roughly 10 pages a month. Individual plans are $16.99/month, or $13.99/month billed annually; Pro is $99.99/month, or $74.99/month annually. Review sites also report a split tiering of $7.99/month for AI detection only, $8.99/month for plagiarism only and $13.99/month combined, plus a $23.99/month Business plan covering 500 pages with cross-language and code scanning. Copyleaks has changed its packaging more than once in 2026 and the figures in circulation conflict; check the pricing page before buying.
2. Scribbr — best for a thesis, dissertation or anything a university will scan
Scribbr is the answer to the specific question most students are actually asking, which is “what will Turnitin say about my paper?” It licenses Turnitin’s technology and matching database, giving an individual access to the paywalled journal and publication content that no web-crawling checker holds. Scribbr states coverage of more than 99 billion web pages and 89 million publications.
One limit to be precise about: Scribbr matches against Turnitin’s web and publications database, not against Turnitin’s repository of previously submitted student papers, which remains institution-only. A Scribbr report is therefore a close approximation of your university’s result rather than an exact preview of it, and it will not catch a passage copied from another student’s unpublished submission.
The privacy design is the part that matters and the part most alternatives get wrong. Scribbr states that documents submitted to its Plagiarism Check and Self-Plagiarism Checker are not stored in any public or shared database, are removed after one year, and are not visible to other plagiarism checkers including the one your university runs — so a pre-check does not cause a 100% self-match when you submit for real. That is the single most consequential feature on this page for a student, and it is the reason to pay for Scribbr rather than run the same document through a free web checker.
Its 88% detection figure is self-published and should be read as a vendor claim. Paperpal’s competing test put Scribbr at 70%.
Pricing: one-off payments, no subscription. $19.95 for documents up to 7,499 words, $29.95 for 7,500 to 49,999 words, $39.95 for 50,000 words and above. Pricing page.
3. Quetext — best free tier, and the clearest reports
Quetext has the most genuinely usable free plan in the category: three checks of up to 500 words each, with its DeepSearch paraphrase matching included rather than held back for paid tiers. For checking a paragraph you are unsure about, it is the fastest honest answer available at no cost. Paid plans add file uploads, PDF export, AI detection, a grammar and spell checker, a citation generator and bulk document scanning.
Two caveats. Quetext publishes no accuracy figure of its own, though Reedsy’s five-tool test recorded it at 63.6% — fourth of the five, ahead of Grammarly and behind Scribbr and QuillBot (tied at 72.7%). And its published pricing is inconsistent across listings: $8.25/month for Essential and $16.50/month for Professional on annual billing are the most commonly cited figures, while other listings report $15.99/month for Essential on monthly billing. Read the pricing page before committing.
Pricing: free tier of three checks at 500 words each. Essential from $8.25/month billed annually; Professional from $16.50/month billed annually. Team and school plans quoted on request.
4. QuillBot — best for students who need citations as well as a score
QuillBot was Reedsy’s overall pick at 8.5 out of 10 despite placing second on raw detection at 72.7%, and the reason is the workflow: it pairs the scan with citation generation and paraphrasing in one product, which is what a student writing a paper actually needs. If you are choosing one subscription to cover checking, citing and rewriting, this is the strongest single purchase on the page.
The trade-off is that the checker is Premium-only and metered. Free accounts cannot run a scan at all. There is also an obvious tension in buying paraphrasing and plagiarism detection from the same vendor, which QuillBot does not address directly.
Pricing: Premium is $19.95/month, $39.95 quarterly (about $13.31/month) or $99.95/year, as listed in June 2026. Premium includes 25,000 words of plagiarism scanning a month, with additional words purchasable. Pricing page.
5. Originality.ai — best for publishers, agencies and SEO teams
Originality.ai is built for people checking other people’s writing at volume rather than checking their own. It combines plagiarism scanning with AI detection, readability, grammar and fact-checking, adds team management and a WordPress plugin, and is the only tool here with a serious pay-as-you-go option — useful if your volume is lumpy. On the AI-detection side it has the best independent result of the mainstream suites: 0.69 overall accuracy against Turnitin’s 0.61 in the 2026 International Journal for Educational Integrity study, though that study also found both tools weak on mixed human-and-AI text.
It publishes no headline plagiarism-detection figure and was not in any of the 2026 comparisons, so its similarity matching is unmeasured by anyone but itself. It is also the wrong tool for a student: it is priced and shaped for editorial teams, and it does not offer the university-database coverage a thesis needs.
Pricing: pay-as-you-go at a one-off $30 for 3,000 credits, where one credit is 100 words for an AI-only scan and two credits per 100 words for AI plus plagiarism; credits expire after two years. Pro is $14.95/month, or $12.95/month annually, for 2,000 credits a month. Enterprise is $179/month, or $136.58/month annually, for 15,000 credits with API access. Pricing page.
6. Grammarly — the convenient option, and the weakest detector tested
Grammarly is on this page because it is where millions of people will look first, and the finding is that they should look somewhere else for this specific job. Reedsy recorded 54.5% detection, the lowest of the five tools it tested and roughly 27 points behind Copyleaks. The plagiarism checker is also gated to the Pro plan, so the effective price of a scan is a full writing-assistant subscription you may not otherwise want.
Where it earns its place is integration. If you already pay for Grammarly Pro and want a rough originality signal inside the editor you are already writing in, it costs nothing extra and takes no extra step. Do not submit academic or professional work on the strength of a Grammarly result alone.
Pricing: plagiarism checking requires Pro. Pro is $30/month billed monthly, about $20/month billed quarterly, or $12/user/month billed annually. Not available on the free plan. Pricing page.
7. Turnitin — the institutional standard you cannot buy
Turnitin is the tool most academic work is judged by and the tool almost no reader of this page can purchase. Turnitin sells to institutions; its help centre states plainly that it does not sell individual or single-use licences, and points individuals to iThenticate instead. Institutional contracts are shaped by user volume, feature bundle, deployment model and support, and there is no published list price.
What it has that nothing else does is the corpus. Turnitin matches against archived student paper submissions — including papers submitted by other students at other institutions — as well as web pages and paywalled journal content. Published figures vary by source and by year: Scribbr, which resells the database, states more than 99 billion web pages and 89 million publications, while figures of roughly 1.9 billion archived student papers and 170 million paywalled articles are widely quoted from Turnitin materials. We have not been able to reconcile these against a single current Turnitin source, so treat the exact numbers as approximate and the relative advantage as real.
On the AI side, Turnitin says its detector exceeds 98% accuracy on documents where more than 20% of the content is AI-generated, with a false-positive rate under 1% at that threshold, and that it updated the model in February 2026 to improve recall and again in July 2026 to consolidate its ensemble into a single model. Reports are not recalculated retrospectively; a paper must be resubmitted to be scored by an updated model.
Pricing: institution-only, quoted on request. Data not available publicly.
8. iThenticate — best for journal submission and research manuscripts
iThenticate is Turnitin’s product for researchers and publishers, and it is the one a manuscript will be run through before peer review. It matches against scholarly content rather than student papers, which is the right index for a paper heading to a journal, and it is the engine behind Crossref Similarity Check, the service most publishers use.
iThenticate 2.0 added AI writing detection as an add-on, and its model was updated to catch text passed through AI bypasser tools, which it now reports inside the AI-generated category rather than treating as human.
Pricing: Turnitin’s own help centre points individuals to iThenticate from $125 per check, which is the figure we use here; some third-party listings still quote $100, so treat that as stale. One document credit covers a file of 25,000 words or fewer and includes five revisions, and credits are valid for 12 months (vendor site). For publishers, Crossref Similarity Check charges an annual service fee of 20% of the member’s Crossref membership fee plus a per-document checking fee, with the first 100 documents free. Crossref does not publish the per-document rate; data not available.
9. Free web checkers — Smodin, DupliChecker, SmallSEOTools and PaperRater
These tools have one job: telling you, at no cost, whether an obvious lift is an obvious lift. On the published evidence they do not do much more than that.
Smodin is the most usable of the group, allowing 1,500 characters per check across five daily scans on the free tier, with paid plans for higher volume. It claims 96% accuracy for its AI content detection; it publishes no plagiarism figure.
DupliChecker offers a 1,000-word limit without registration, double what most free tools allow. In a 15-tool comparison published by bestwriting.com it returned 7% against a passage other tools flagged at between 25% and 79%, catching one source and missing the rest. SmallSEOTools also allows 1,000 words and, in the same comparison, marked as fully unique a passage other tools flagged heavily. PaperRater detected none of the plagiarised content in that test.
That comparison is affiliate-funded and has not been reproduced, so read the individual figures as indicative rather than measured. The pattern it describes — that free checkers under-detect badly, with the same comparison putting free-tool detection at around 43% against 88% for its top paid performer — is consistent with the structure of the market, because index size is what costs money.
Use them for: a quick sanity check on a paragraph. Do not use them for: anything that will be graded, published or paid for.
10. Code plagiarism — Codequiry, MOSS and Copyleaks
Prose checkers do not work on source code, because code has a small vocabulary, enforced structure and heavy legitimate reuse. Three options cover it.
MOSS, Stanford’s Measure of Software Similarity, is free, is the long-standing academic default, and compares submissions within a batch against each other. Its structural limitation is that it does not check the open web, so a solution copied from GitHub or a tutorial and submitted by one student in a cohort will not be flagged.
Codequiry exists to close that gap: it checks submissions against each other, against the web including public repositories, and for AI-generated code. Pricing starts at $29/month, with plans for teachers from $79/month.
Copyleaks includes source-code scanning on its Business tier, which makes it the sensible choice if you are already buying it for prose.
Automated obfuscation is the live problem here. A May 2025 arXiv study, “Evaluating Software Plagiarism Detection in the Age of AI”, tested how well code-similarity systems hold up when a language model is used to restructure a submission, and the results are a caution against treating any code-similarity score as conclusive.
Discontinued: Unicheck
Unicheck was acquired by Turnitin and discontinued on 1 January 2025. It still appears in comparison articles and procurement documents. It is not a purchasable option in 2026, and its LMS integrations for Google Classroom, Canvas, Moodle and Blackboard are now served by Turnitin’s own products.
Feature comparison: the full matrix
| Tool | Free tier | Entry paid price | AI detection | Academic database | Stores your document | Code | Published detection rate |
|---|---|---|---|---|---|---|---|
| Copyleaks | About 10 pages/month | $13.99/month annually | Yes | No | No public database | Yes, Business tier | 81.8% (Reedsy) |
| Scribbr | No | $19.95 one-off | Separate product | Yes, via Turnitin | No, deleted after one year | No | 72.7% (Reedsy); 88% (self) |
| Quetext | 3 checks, 500 words each | $8.25/month annually | Yes, paid tiers | No | Not published | No | 63.6% (Reedsy) |
| QuillBot | No checker on free plan | $19.95/month | Yes | No | Not published | No | 72.7% (Reedsy) |
| Originality.ai | No | $30 one-off, or $12.95/month | Yes | No | Not published | No | None published |
| Grammarly | No checker on free plan | $12/user/month annually | Yes | No | Not published | No | 54.5% (Reedsy) |
| Turnitin | No | Institution-only quote | Yes, add-on | Yes | Yes, student paper repository | No | None published for similarity |
| iThenticate | No | From $125 per manuscript | Yes, add-on | Yes, scholarly content | Configurable by account | No | None published |
| Winston AI | 14-day trial | $18/month | Yes | No | Not published | No | None published |
| Smodin | 1,500 characters, 5 scans/day | Paid plans available | Yes | No | Not published | No | None published |
| DupliChecker | 1,000 words | Free only | Limited | No | Not published | No | 7% in one affiliate test |
| Codequiry | No | $29/month | Yes, AI code | No | Not published | Yes | None published |
| MOSS | Free | Free | No | No | Submissions retained | Yes | None published |
“Stores your document” is the column students should read first. Turnitin’s institutional product adds submitted papers to a repository other institutions match against, by design. Scribbr explicitly does not. Most commercial tools do not publish a clear answer, which is itself an answer for anything confidential.
Pricing comparison: what you will actually pay
| Tool | Free tier | Cheapest paid option | Volume or top tier |
|---|---|---|---|
| Quetext | 3 checks of 500 words | $8.25/month annually | Professional $16.50/month annually |
| Copyleaks | About 10 pages/month | $13.99/month annually | Pro $74.99/month annually |
| Originality.ai | None | $30 one-off for 3,000 credits | Enterprise $179/month |
| Grammarly | No plagiarism check | $12/user/month annually | $30/month billed monthly |
| Winston AI | 14-day trial | $18/month | $49/month |
| QuillBot | No plagiarism check | $99.95/year | $19.95/month |
| Scribbr | None | $19.95 per document to 7,499 words | $39.95 for 50,000+ words |
| Codequiry | None | $29/month | Teachers from $79/month |
| iThenticate | None | From $125 per manuscript | Publisher accounts via Crossref |
| Turnitin | None | Institution-only quote | Data not available |
| MOSS | Free, academic use | Free | Free |
The structural point: subscriptions suit people who check continuously, and one-off pricing suits people who check a thesis once. A student paying $19.95 to Scribbr twice a year spends less than a third of what a $12/month Grammarly Pro subscription costs over the same period, and gets the database that matters.
Which plagiarism checker should you use?
Best plagiarism checker overall
Copyleaks. It posted the highest detection rate, at 81.8%, in the one 2026 comparison not published by a competing vendor, covers more than 100 languages with cross-language matching, and handles code as well as prose.
Best for a thesis, dissertation or graded coursework
Scribbr. It is the only individually purchasable tool running on Turnitin’s matching technology and publications database — though not on Turnitin’s student-paper repository — and it states that your document is not added to any shared database and is deleted after a year — so checking first cannot create a self-match when you submit. Pay per document rather than monthly.
Best free plagiarism checker
Quetext. Three checks of 500 words each with paraphrase matching included, which is more usable than any other free tier here. Smodin is the alternative at 1,500 characters across five scans a day. Neither is adequate for a full document.
Best for publishers, agencies and content teams
Originality.ai. Team management, a WordPress plugin, API access on Enterprise, pay-as-you-go credits for uneven volume, and the strongest independent AI-detection result of the mainstream suites. For the wider content workflow, see best AI for SEO.
Best for students who also need citations
QuillBot. Reedsy’s overall pick at 8.5 out of 10, pairing 72.7% detection with citation generation in one subscription. See also best AI study tools and best AI for students.
Best for a journal manuscript
iThenticate. It matches against scholarly content rather than student work and is the engine behind Crossref Similarity Check, so it approximates what a publisher will run. Budget from $125 per manuscript.
Best for schools and universities
Turnitin, on the strength of the student-paper repository and LMS integration, with Copyleaks the credible alternative where multilingual coverage, code scanning or cost matter more than the archive. Institutions piloting AI policy should look at Turnitin Clarity, which records the writing process rather than scoring the finished text. See also best AI for teachers.
Best for source code
Codequiry if you need web and repository matching plus AI-code detection, MOSS if a free within-cohort comparison is enough. Related: best AI code review tools.
The tools to be most careful with
Free web checkers. In the only published test of them we found, DupliChecker returned 7% on a passage others flagged between 25% and 79%, SmallSEOTools called a heavily flagged passage unique, and PaperRater detected nothing. A “0% plagiarism” result from a free tool is not a clearance.
What a similarity score actually means
The number a checker returns is the proportion of your text that matched something in its index. It is not a plagiarism percentage, and no threshold in it constitutes proof.
Four things routinely inflate it without any wrongdoing: correctly quoted and attributed passages, the reference list, standard methodological or legal boilerplate, and common phrases that appear in millions of documents. Most tools can exclude quotations and bibliographies from the count, and most institutional configurations do. A 20% score composed of correctly cited quotations is fine; a 6% score that is one uncited paragraph lifted verbatim is not.
Two things routinely deflate it. Paraphrased material that preserves the source’s structure and argument while changing the words may match nothing at all and still be plagiarism in every sense a marker cares about. And material from a source the tool has not indexed — a paywalled book, an unpublished paper, another student’s unsubmitted work — cannot be matched at all.
The practical rule: read the matched sources, never the headline number. A similarity report is useful precisely because it shows you the document it matched against, which is the one thing an AI detector can never give you.
Where plagiarism checkers break
- Paraphrase. This is the structural weakness of the whole category. String matching cannot see meaning, and modern paraphrasing defeats it deliberately; the “All That Glitters is Not Novel” work documents language models generating paraphrased papers shaped to avoid detection. Semantic and cross-language matching narrows the gap but does not close it.
- Index gaps. A checker finds only what it has crawled. Paywalled journals, books, institutional repositories and other students’ papers are behind the pricing wall of exactly one or two vendors, which is the whole reason Turnitin’s position is durable.
- AI-generated text. Text produced fresh by a model is genuinely novel string-by-string and scores near zero. This is not a bug in similarity matching; it is a category error to expect otherwise. Use an AI detector for that question, with all of the accuracy caveats that come with it.
- Translated text. Material translated from another language matches nothing in the original language’s index unless the tool does cross-language comparison, which Copyleaks does and most free tools do not.
- Code. Prose tools cannot handle source code, and even dedicated code tools degrade when a model is used to restructure a submission, as the May 2025 obfuscation study shows.
- Self-plagiarism against unindexed work. Reusing your own earlier submission is caught only if that submission is in the index, which is exactly why institutional repositories exist and why a private pre-check is worth paying for.
Recent developments (2025–2026)
- 1 January 2025 — Unicheck discontinued. Turnitin retired the product it acquired; its LMS integrations moved to Turnitin’s own line (Unicheck).
- 4 March 2025 — Turnitin Clarity announced at SXSW EDU, shifting the evidence from the finished text to the writing process — total writing time, edit history and pasted text (Turnitin).
- May 2025 — software-plagiarism obfuscation study published, testing how code-similarity detection holds up against AI-driven restructuring (arXiv).
- February 2026 — Turnitin updates its AI writing model to improve recall while holding false positives low, and upgrades detection of AI “bypasser” or humaniser tools. Existing reports are not recalculated; papers must be resubmitted (Turnitin).
- Spring 2026 — Turnitin Clarity enters institutional pilots, including at the University of Arkansas (TIPS).
- 2026 — Turnitin reports 14.8% of English-language submissions between October 2025 and February 2026 contained more than 80% AI-generated writing, against 3.3% between April and August 2023 (Turnitin).
- 2026 — International Journal for Educational Integrity publishes two detector evaluations, one finding Originality.ai ahead of Turnitin on AI-detection accuracy at 0.69 against 0.61 (IJEI), the other finding Pangram the most reliable of the tools tested (IJEI).
- July 2026 — Turnitin consolidates its AI detection ensemble into a single model, reporting a simplified AI writing report and a false-positive rate below 1% (Turnitin).
- 2026 — iThenticate 2.0 updates its AI writing model to flag bypasser-modified text, reporting it within the AI-generated category (iThenticate).
Frequently asked questions
What is the best plagiarism checker in 2026?
Copyleaks is the best general-purpose choice, because it recorded the highest detection rate at 81.8% in Reedsy’s 2026 five-tool comparison, the only such test published in 2026 that we found was not run by a company selling one of the tools in it. For academic work the better answer is Scribbr, because it licenses Turnitin’s matching technology and publications database, which is much closer to what a university will match your work against than any web-crawling tool. Quetext is the best free option at three checks of 500 words each.
Are plagiarism checkers accurate?
Less accurate than their marketing implies, and less well measured than AI detectors. The three published 2026 comparisons recorded detection rates between 54.5% and 90%, disagreed with each other, and two of the three were run by companies selling a checker in their own ranking. No peer-reviewed independent benchmark of similarity matching was published in 2026. Accuracy also depends almost entirely on index coverage rather than on the algorithm, which is why the same passage scores very differently across tools.
What is the best free plagiarism checker?
Quetext has the most usable free tier, at three checks of up to 500 words each with its DeepSearch paraphrase matching included. Smodin allows 1,500 characters across five scans a day. Beyond those, free tools under-detect badly: in a 15-tool comparison published by bestwriting.com, DupliChecker returned 7% on a passage other tools flagged between 25% and 79%, and PaperRater detected none of it. A clean result from a free checker is not a clearance for graded or published work.
Can students buy Turnitin?
No. Turnitin’s help centre states that it does not sell individual or single-use licences, and directs individuals to iThenticate, which starts at $125 per check. There is no published list price for institutional Turnitin; contracts depend on user volume, feature bundle and deployment model. The nearest cheap individual equivalent is Scribbr, which licenses Turnitin’s matching technology and publications database and charges $19.95 to $39.95 per document.
Will using a plagiarism checker before submission cause a 100% match later?
It depends entirely on the tool, and this is the single most important thing to check before uploading. Scribbr states that documents are not stored in any public or shared database, are removed after one year, and are not visible to other checkers including the one your university uses. Turnitin’s institutional product does add submitted papers to a repository other institutions match against, by design. Most commercial tools do not publish a clear policy, so read the privacy terms before uploading anything confidential.
Do plagiarism checkers detect AI-written text?
Not as plagiarism, no. Text generated fresh by a language model is novel string-by-string and typically returns a near-zero similarity score, because there is no source document to match against. Most checkers now bundle a separate AI-detection feature that estimates whether text was machine-written, and that is a different technology with different failure modes — see our AI detector rankings. A low similarity score is not evidence that a person wrote the text.
What is a good similarity score?
There is no threshold that means plagiarism, and any tool or policy implying otherwise is oversimplifying. The score is the proportion of text matching the tool’s index, and it is inflated by correctly cited quotations, reference lists and standard boilerplate, most of which can be excluded from the count. A 20% score made up of properly attributed quotations is fine; a 6% score containing one uncited verbatim paragraph is not. Read the matched sources, not the headline number.
Can plagiarism checkers detect paraphrased text?
Partially, and this is the category’s structural weakness. Exact string matching cannot see meaning, so paraphrase that preserves an argument while changing the words may match nothing. Vendors have responded with semantic and cross-language matching — Copyleaks and Quetext’s DeepSearch both target rewritten text — but research such as “All That Glitters is Not Novel” documents language models producing paraphrased papers specifically shaped to evade detection. Institutions increasingly pair similarity matching with AI detection for this reason.
Is Grammarly’s plagiarism checker any good?
It was the weakest of the five tools in Reedsy’s 2026 comparison, at 54.5% detection against Copyleaks’ 81.8%. It also requires a Grammarly Pro subscription, which is $12 per user per month billed annually or $30 billed monthly, so it is not a cheap way to buy a scan. It is reasonable as a convenience check inside an editor you already pay for, and it is not adequate on its own for academic or professional submission.
What is the difference between Turnitin and iThenticate?
They are both Turnitin products with different indexes and different buyers. Turnitin serves education and matches against archived student papers plus web and journal content, integrated into learning management systems. iThenticate serves researchers and publishers, matches against scholarly content rather than student work, and is the engine behind Crossref Similarity Check. If your document is going to a journal, iThenticate approximates what the publisher will run; if it is going to a marker, Turnitin does.
Do plagiarism checkers work on source code?
Not the prose tools. Code has a small vocabulary, enforced syntax and legitimate reuse, so text-similarity engines produce noise. MOSS is the free academic default and compares submissions within a cohort against each other, but does not search the open web, so code copied from a public repository by one student will not be flagged. Codequiry adds web and repository matching plus AI-code detection from $29/month, and Copyleaks includes code scanning on its Business tier.
How much does a plagiarism checker cost?
For an individual, between nothing and about $40 per document. Quetext starts at $8.25/month billed annually, Copyleaks at $13.99/month billed annually, and Originality.ai offers $30 pay-as-you-go for 3,000 credits. Scribbr charges one-off fees of $19.95, $29.95 or $39.95 depending on word count, which is usually cheaper than a subscription if you check a document once or twice a year. Institutional Turnitin pricing is quoted per contract and is not published.
Can a plagiarism checker be wrong?
Routinely, in both directions. False positives come from correctly cited quotations, reference lists and common phrasing being counted in the score. False negatives come from paraphrase the tool cannot see and from sources it has not indexed — a checker cannot find what it has never crawled. Neither a high score nor a clean report is a conclusion. The report is a list of things to look at, and the judgement belongs to a person who reads the matched sources.
Conclusion: how to choose in August 2026
Three decisions cover almost every case.
- If the work will be marked or examined, buy the database, not the subscription. Scribbr at $19.95 to $39.95 per document gives you Turnitin’s matching technology and publications database without adding your paper to anyone’s repository. Nothing that crawls only the open web is a substitute.
- If you check continuously, buy Copyleaks. The best detection rate on the least conflicted evidence available, plus more than 100 languages, cross-language matching and code coverage, from $13.99/month billed annually.
- If you just need to check a paragraph, Quetext is free and honest about its limits. The other free tools under-detect badly enough that a clean result from them tells you nothing.
And the caveat worth carrying away from this page: the plagiarism-checker market publishes far less credible accuracy evidence than the AI-detection market next door, and the numbers that do exist mostly come from companies ranking themselves. Choose on index coverage and document-storage policy, which are verifiable, rather than on accuracy percentages, which largely are not.