Quick Answer
The best free AI detector depends on the job: GPTZero allows 10,000 words a month, QuillBot and Scribbr check 1,200 words per scan, and Copyleaks scans 25,000 characters without an account. No free AI detector can prove authorship, because a peer-reviewed study of 14 detection tools found none reached 80% accuracy.
Key Takeaways
- GPTZero’s free plan covers 10,000 words per month, with up to 10,000 characters per scan and sentence-level highlighting.
- Scribbr offers unlimited free checks of up to 1,200 words with no sign-up, while QuillBot caps free users at 6 scans per day.
- A 2023 study in the International Journal for Educational Integrity tested 14 tools and found none above 80% accuracy.
- Stanford researchers found 7 detectors flagged an average of 61.3% of essays by non-native English writers as AI-generated.
- OpenAI shut down its own AI text classifier on July 20, 2023, citing its low rate of accuracy.
What is the best free AI detector?
The best free AI detector is the one whose free limits match the length of text you need to check, because the major tools use similar detection methods and publish similar vendor accuracy claims. GPTZero suits people who check many medium-length documents over a month. Scribbr and QuillBot suit single essays under 1,200 words. Copyleaks and ZeroGPT suit longer one-off checks because they accept more characters per scan without payment.
This comparison is based on each vendor’s published free-tier limits and on independent studies, not on hands-on testing. That distinction matters because every vendor in this list advertises high accuracy, and those figures are self-reported. The practical approach is to choose a detector by its limits and features, then treat its score as one signal rather than a verdict. Readers who want free writing assistants rather than detectors can compare options in our guide to the best free ChatGPT alternatives.
How do the free AI detectors compare?
The 8 free AI detectors below differ most in scan length, daily or monthly caps, and whether an account is required. The figures come from each vendor’s official product or pricing page as of September 2026. Accuracy claims are listed as the vendor’s own and have not been independently confirmed.
| Detector | Free limit (as of September 2026) | Account needed? | Vendor accuracy claim (self-reported) |
|---|---|---|---|
| GPTZero | 10,000 words per month; 10,000 characters per scan | Only for scans over 10,000 characters | 99% accuracy |
| QuillBot | 1,200 words per scan; 6 scans per day; 80-word minimum | Not stated | 99% detection rate on the RAID benchmark |
| Scribbr | Unlimited checks of up to 1,200 words each | No | No figure published |
| ZeroGPT | 15,000 characters per scan | Not stated | 98.4% accuracy, under 1% false positives |
| Copyleaks | 25,000 characters without logging in; 255-character minimum | No (a free account adds credits) | Over 99% accuracy |
| Originality.ai | 3 scans per day; 2,000 words per scan | No | 97.8% for its multilingual model |
| Sapling | 2,000 characters per check | Not stated | No figure published |
| Grammarly | Free web detector; limits not published | Not stated | 99% detection accuracy |
Turnitin is not on this list because it has no free option for individuals. Turnitin sells its AI writing indicator to schools, and only instructors and administrators can see the result. Students cannot run their own work through the same model before submission, which is one reason many students use the free tools above to check drafts.
Which free AI detector is best for students and teachers?
GPTZero and Scribbr are the most practical free AI detectors for classroom use. GPTZero’s free plan provides 10,000 words per month and highlights the sentences it considers most likely to be AI-written, according to GPTZero’s pricing page. GPTZero states that it fully supports English, German, Portuguese, French and Spanish, and its own FAQ says results should not be used to punish anyone or as a final verdict.
Scribbr allows unlimited free checks of up to 1,200 words per submission without sign-up, which covers a typical short essay. Scribbr separates results into human-written, AI-generated and AI-refined text and gives paragraph-level feedback. Scribbr’s page also states that no AI detector can provide complete accuracy, and it now points users who want a more detailed check to QuillBot’s detector.
Teachers should read the limitations section below before using any score in a grading decision. Many schools now publish their own AI policies, and some districts have adopted AI tools directly, as covered in our report on ChatGPT rolling out to teachers in school districts. Students looking for approved study aids can also review the best AI tools for students.
Which free AI detector handles the longest text?
Copyleaks handles the longest single free check among these tools, at up to 25,000 characters without logging in, which is roughly 3,500 to 4,500 words of English prose. Copyleaks’ pricing page lists the Personal plan at $13.99 per month billed annually ($167.88 per year) for 1,200 credits, which Copyleaks says covers up to 300,000 words. Copyleaks states that it detects AI text in more than 30 languages.
ZeroGPT accepts 15,000 characters per free scan and highlights each sentence it attributes to AI. ZeroGPT’s FAQ recommends at least 150 to 200 words for a meaningful result. GPTZero’s per-scan limit is 10,000 characters, although its monthly allowance is larger than most competitors’ free tiers.
Longer text is not automatically more reliable. Detectors are generally more confident on long passages than on short ones, but a long document that mixes human writing, AI drafting and human editing produces a blended score that is harder to interpret. The most sensible approach is to check suspicious sections separately and compare the highlighted sentences with the writer’s earlier work.
Which free AI detectors have strict daily limits?
QuillBot and Originality.ai have the strictest daily caps among the free options. QuillBot’s AI detector page lists up to 1,200 words per scan, up to 6 scans per day and a minimum of 80 words. QuillBot claims a 99% detection rate based on the RAID benchmark (self-reported), and QuillBot itself warns never to make an academic or employment decision on a detector score alone.
Originality.ai’s pricing page allows 3 free scans per day of up to 2,000 words each, limited to AI detection. Originality.ai notes that text submitted on the free tier may be used for training, and account holders can opt out. That detail matters for anyone checking unpublished work, client documents or student essays that contain personal information. Originality.ai’s paid Pro plan costs $12.95 per month on annual billing, with 1 credit equal to 100 words.
Sapling’s free detector is limited to 2,000 characters per check, which is roughly 300 to 400 words. Sapling says its detector becomes much more accurate after about 50 words and states that no current AI content detector, including its own, should be used as a standalone check.
Are AI detectors accurate?
AI detectors are not accurate enough to prove that a person used AI. A 2023 study by Weber-Wulff and colleagues in the International Journal for Educational Integrity tested 14 detection tools, including Turnitin, and found that even the highest accuracy values were below 80%. The researchers concluded that the tools were neither accurate nor reliable, and accuracy fell further when AI text was edited by hand or run through a paraphrasing tool.
Later research reached similar conclusions. A 2024 study by Perkins and colleagues in the International Journal of Educational Technology in Higher Education ran 805 tests on 6 detectors and found a mean accuracy of only 39.5% on unedited AI-generated text. Simple evasion techniques reduced accuracy by a further 17.4 percentage points on average. Human-written control samples were classified correctly 67% of the time.
Even the company behind ChatGPT could not build a reliable detector. OpenAI’s AI text classifier launched on January 31, 2023, correctly identifying 26% of AI-written text and wrongly labeling human text as AI-written 9% of the time. OpenAI discontinued the classifier on July 20, 2023, due to its low rate of accuracy. This history explains why vendor claims of 98% or 99% accuracy should be read as results on the vendor’s own test data rather than as a guarantee for any single document.
Why do AI detectors flag human writing?
AI detectors flag human writing mainly because they measure how predictable the text is, and plain, formulaic or simplified writing looks predictable. A 2023 Stanford study published in Patterns tested 7 detectors on 91 TOEFL essays written by non-native English speakers. The detectors wrongly flagged an average of 61.3% of those human essays as AI-generated, and all 7 detectors agreed on 19.8% of them.
The same study found that rewriting the essays with richer vocabulary cut the false-positive rate to 11.6%. This result shows that detectors can react to vocabulary and style rather than to authorship. Non-native English writers, students writing to a strict template and people using grammar tools that simplify phrasing all face a higher risk of a false flag.
Institutional detectors also produce false positives. Turnitin reports a document-level false positive rate under 1% for documents with 20% or more AI writing and a sentence-level rate of around 4%, and Turnitin’s guidance says its indicator should not be used as the sole basis for action. Vanderbilt University disabled Turnitin’s AI detector on August 16, 2023, calculating that a 1% false positive rate across 75,000 papers could have wrongly labeled around 750 student papers.
How can you tell if text is AI generated?
To tell if text is AI generated, combine a detector score with evidence about how the text was written, because no single signal is conclusive. The steps below reflect the guidance published by vendors and universities rather than a single test.
- Run the text through 2 different free detectors and compare the highlighted sentences, not only the overall percentage.
- Check that the sample meets each tool’s minimum length, such as QuillBot’s 80 words, because short passages produce unreliable results.
- Compare the text with the writer’s earlier work for changes in vocabulary, structure and typical errors.
- Ask for process evidence, such as drafts, notes, outlines or the document’s version history in Google Docs or Microsoft Word.
- Discuss the work with the writer and ask them to explain their sources and reasoning in their own words.
Stop at the discussion stage if the only evidence is a detector score. A detector result alone should not lead to a grade penalty, a disciplinary report or a rejected job application, and most vendors in this article explicitly advise against that use. Students who are accused on the basis of a score should ask which tool was used, request the full report and share their drafts and version history with the instructor or academic integrity office.
Can AI watermarks replace AI detectors?
AI watermarks cannot replace general AI detectors yet, because a watermark only identifies text from the company that added it. Google’s SynthID watermarks text generated by Google’s AI tools, and Google open-sourced SynthID Text in October 2024. Google announced a SynthID Detector portal in May 2025 and, on October 7, 2026, opened it to everyone globally in English at synthid.com. Google describes the public portal as a way to check whether an image, video or audio file was made with AI from Google or its partners, including OpenAI, NVIDIA and Kakao, and the announcement does not list text among the supported file types. A SynthID check therefore still cannot identify text from ChatGPT, Claude or other non-Google models.
Google states that SynthID’s detection confidence can fall sharply when AI-generated text is thoroughly rewritten or translated. For now, watermarking is one more signal for content from a specific provider rather than a universal test. Readers comparing how the major chatbots handle schoolwork can see our comparison of Gemini vs ChatGPT for students.
For free options in other categories, see our guide to the best free AI tools, which picks the strongest free tool for chat, images, research, coding, voice, music and slides.
Frequently Asked Questions
Is there a completely free AI detector with no word limit?
No free AI detector offers unlimited words per scan. Scribbr comes closest with unlimited free checks of up to 1,200 words each, while GPTZero allows 10,000 words per month on its free plan.
Which free AI detector is the most accurate?
No free AI detector has been independently proven to be the most accurate, because published accuracy figures come from the vendors themselves. Independent studies found that none of 14 tested tools exceeded 80% accuracy, so results from any detector should be treated as one signal.
Can AI detectors be wrong about human writing?
Yes, AI detectors regularly flag human writing as AI-generated. A 2023 Stanford study found 7 detectors wrongly flagged an average of 61.3% of essays by non-native English writers.
Can students use Turnitin’s AI detector for free?
No, Turnitin’s AI writing detector is not available free to individual students. Turnitin licenses it to institutions, and only instructors and administrators can see the AI writing indicator.
Do AI detectors work on paraphrased text?
AI detectors work poorly on paraphrased text. The 2023 Weber-Wulff study found that most AI-generated texts remained undetected after machine paraphrasing, and a 2024 study found simple evasion techniques cut detector accuracy by 17.4 percentage points on average.