The short answer
Not reliably enough to use alone. Grammarly says its AI detector reaches 99% accuracy and ranks first on the RAID benchmark, but it publishes no false positive rate, and the only independent test of its free checker caught none of nine AI samples. Treat a Grammarly score as a prompt to reread, not proof.
How accurate is Grammarly's AI detector?
Its accuracy on student essays is unproven. Grammarly makes a strong claim, a research leaderboard partly backs it, and every published hands-on test was run by a company that sells a competing detector. Plagino has not benchmarked Grammarly either, so this page separates what Grammarly says, what others measured, and what we measured ourselves.
What Grammarly claims
Grammarly's AI detector page says the tool "achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark". The same page says "no AI detector is 100% accurate" and that detection should never be used as a standalone verification method. Its user guide goes further: the percentage "should not be used as an objective source of truth".
Grammarly does not publish a false positive rate, a per-model detection rate, or the leaderboard score behind the 99% figure. Its February 2026 announcement says it "ranked #1 on RAID's leaderboard for AI detection quality" but gives no number.
What the RAID ranking does and does not measure
RAID is a research benchmark from University of Pennsylvania researchers, published at ACL 2024. It holds over 6 million generations from 11 language models across 8 domains, 11 adversarial attacks and 4 decoding strategies. A leaderboard rank there is a meaningful result.
It is not a test of student writing, though. RAID reports the share of AI text caught while the false positive rate is held at a fixed 5%, which is a research convention rather than "accuracy". Its text was generated in 2023, from models up to GPT-4, and includes no Claude or Gemini output. The RAID authors themselves conclude that current detectors are "easily fooled" by adversarial attacks, sampling changes and unseen models. So the 99% claim tells you Grammarly does well on that corpus, not how it scores a 2026 essay written with a newer chatbot.
What independent tests found
Every hands-on test we could find was small, and each was run by a competitor. The table lists them with their sample sizes, because the sample size matters more than the headline.
| Test (date) | Who ran it | Sample | Grammarly result | Conflict of interest |
|---|---|---|---|---|
| Pangram 30-tool test (7 Jan 2026) | Pangram Labs | 9 AI texts (GPT-4o, Gemini 2.0, Claude 3.7 Sonnet) and 3 human texts, free tier | 0 of 9 AI texts flagged at a 75% threshold; 3 of 3 human texts passed | Sells a rival detector |
| RAID subset test (12 Nov 2025) | Originality.ai | 99 RAID samples across 9 attacks and 11 models | Recall 0.222, F1 0.364 | Sells a rival detector |
| Three ChatGPT samples (12 Nov 2025) | Originality.ai | 3 unedited ChatGPT texts | 99%, 93% and 71% AI | Sells a rival detector |
| One blog post (24 Apr 2025) | Quetext | 1 unedited ChatGPT text | 10% AI | Sells a rival detector |
Read together, the tests point one way: Grammarly tends to catch raw, unedited AI text and tends to miss text that is newer, polished or deliberately altered. None of them is large enough to estimate a rate, and Pangram itself calls its test not definitive. If you need a number you can defend, none of these supplies one.
How does the Grammarly AI checker work?
Grammarly splits your text into sections, scores each section for patterns common in AI writing, and reports the share of the text that "appears to be AI-generated". The model is proprietary. Grammarly's detector page says it was trained on "tens of thousands of texts", including human and AI text created before 2021, and that it "updates continuously".
What the percentage means
The percentage is an estimate of how much of the text looks AI-generated, not the probability that you cheated. Grammarly's user guide calls its scores "average estimates" and says shorter passages are harder to score accurately. Grammarly does not publish a minimum or maximum length.
One detail in the guide matters for students. Grammarly says using Grammarly or other AI tools to meaningfully rewrite text raises the score. A draft you wrote yourself and then rephrased with an AI feature can score higher than the draft you started with.
Free AI checker vs the AI Detector agent
Grammarly runs two different products under the AI detector name, and reviews often mix them up.
| Free AI checker | AI Detector agent | |
|---|---|---|
| Where | grammarly.com/ai-detector | Inside Google Docs, Word and Grammarly's editor |
| Cost | Free | Paid plans (Pro and above) |
| Input | Paste text or upload a file | The document you are writing |
| Output | One percentage, with sections labelled "Resembles AI text" or "No AI text patterns found" | Percentage plus the parts that may sound AI-generated |
The free checker is the version Pangram tested. The agent is a writer's self-check: it shows you your own score while you draft and does not report to an instructor.
Why does Grammarly flag human writing as AI?
Human writing gets flagged when its patterns, syntax and complexity resemble AI text. Grammarly's user guide admits this happens, calls it very rare, and says the model is tuned to limit false positives. It gives no rate, so "very rare" cannot be checked.
Formal, formulaic and second-language writing is most exposed with any detector. A 2023 Stanford study found that several AI detectors labelled more than half of the TOEFL essays it tested as AI-generated. That study did not test Grammarly, but it shows why a published false positive rate matters more than a claimed accuracy. In Plagino's March 2026 benchmark, the false positive rate ranged from 0.8% to 6.3% across seven detectors. Our guide to AI detector false positives explains why the spread is that wide.
Will a Grammarly AI score predict what Turnitin shows?
No. Grammarly's user guide says it uses a proprietary in-house model, so its results can differ from Turnitin, GPTZero or Copyleaks on the same text. A 0% from Grammarly does not promise a low Turnitin score, and a high one does not mean Turnitin will flag you.
The reverse question, whether using Grammarly gets you flagged, depends on which feature you used. Turnitin representatives told SUNY New Paltz in 2023 that small changes from an AI-powered grammar check are unlikely to be flagged, but that "bigger rephrasing changes, to several sentences in a row" likely will be (New Paltz faculty notice). Spelling and grammar fixes are low risk. Letting Grammarly's generative features rewrite or draft paragraphs is not. Our guide to whether Turnitin detects AI covers how Turnitin's indicator works.
How do Plagino and Grammarly compare side by side?
They are built for different jobs. Grammarly is a writing assistant that added AI detection and plagiarism checking. Plagino is a pre-submission check that returns one report on a finished draft. The table shows what each does as of October 2026.
| Plagino | Grammarly | |
|---|---|---|
| AI docs caught | 97.4% in Plagino's March 2026 benchmark (n=500) | Not tested by Plagino. Grammarly claims 99% accuracy |
| Human docs wrongly flagged | 0.8% in the same benchmark (n=500) | Not tested by Plagino. No false positive rate published |
| Price | Free: 3 scans a month. Pro: $19 a month | Free AI checker. Pro: $12 a month billed annually, $30 month to month |
| Plagiarism check | Yes, against the CORE open-access research index | Pro only, against web pages and ProQuest academic databases |
| Input | Upload a .docx file | Paste text, upload a file, or check inside Google Docs and Word |
| Output | PDF report with sentence-level AI highlights and matched sources | Percentage score; the paid agent points to flagged parts |
| Writing-process record | No | Yes, through Grammarly Authorship |
Detection rate and false positives: what the numbers say
A detector can fail in two ways: it can miss AI writing, or it can flag a human. For a student, the second failure is the one that leads to an integrity meeting, so we report both separately.
Detection rate
In Plagino's benchmark, run in March 2026 on 500 AI-written documents from GPT-4o, Claude and Gemini (some lightly edited by people), Plagino flagged 97.4%, with a 95% confidence interval of roughly 95.6% to 98.5%. Turnitin did better on detection, at 98.2%. We publish that loss alongside our wins.
For Grammarly, the only detection figure is its own: 99%, tied to the RAID ranking. That figure comes from a corpus with no Claude or Gemini text, while our benchmark used both.
False positive rate
On 500 human-written documents in the same run, Plagino wrongly flagged 0.8% (interval roughly 0.3% to 2.0%), the lowest of the seven tools tested. Turnitin flagged 1.4% and GPTZero 5.7%.
Grammarly publishes no false positive rate. Pangram's test passed all three of its human texts, which is consistent with Grammarly's claim that it avoids false flags but far too small to measure a rate.
What does each tool cost and what does it check against?
As of October 2026, Grammarly's free AI checker costs nothing and needs no plan. The plagiarism check and the in-editor AI Detector agent need Grammarly Pro. Grammarly's plans page lists Pro at $12 a month, which on 3 October 2026 was the annual-billing rate ($144 a year), with $30 for month-to-month billing. Enterprise is priced by quote.
Is Grammarly's plagiarism checker free?
Not the full check. Grammarly's plans page lists plagiarism detection as unavailable on the Free plan and included in Pro. Its plagiarism checker compares text with "over 16 billion web pages and academic papers in ProQuest's databases". That is a broader pool than Plagino's for web content, and a real advantage if your worry is copied web pages.
Plagino is free for 3 scans a month with no card, and Pro costs $19 a month for 20 scans a day. Its plagiarism check matches passages against open-access academic literature through CORE. It does not search the open web, paywalled journals or other students' papers, and it cannot see Turnitin's private repository. Each match points at the exact overlapping words.
Privacy and data use
Grammarly is part of Superhuman, and the Superhuman privacy policy (effective 6 July 2026) says the company does not make money "by selling or monetizing user content" and lets you control whether your content trains its AI models. The policy gives no fixed retention period for user content. Grammarly's plagiarism page says checked text "will never be publicly searchable or stored in any database accessible by others".
Plagino reports go to your own account. A Plagino report, like any detector result, is a prompt to review the writing, not evidence of misconduct.
Which should you use: Plagino or Grammarly?
Use Grammarly while you write and Plagino when the draft is finished. The two overlap less than their names suggest, and many students will reasonably use both.
Where Grammarly is the better choice
If you already pay for Pro, its checks run as you write in the documents you already use, while Plagino needs a finished .docx upload. If your risk is web plagiarism, Grammarly's source pool is wider. And Grammarly is a writing assistant first, which Plagino is not.
Where Plagino is the better choice
Plagino fits when you want one report on a finished draft that shows both kinds of risk. You upload a Word file and get a PDF with sentence-level AI highlights and the academic sources your text overlaps with, which you can keep with your drafts. It is also the only one of the two with a published false positive rate. You can try the AI detector on a draft for free, and students checking a paper before they submit can read how that works on the AI checker for students page.
| Your situation | Better fit | Why |
|---|---|---|
| You already pay for Grammarly Pro | Grammarly | Its AI and plagiarism checks cost nothing extra and run inside Docs and Word |
| You worry about text copied from websites | Grammarly | It searches billions of web pages; Plagino searches open-access research only |
| You want proof of how you wrote something | Grammarly Authorship | It records your process, which a detector cannot |
| You want grammar and tone help | Grammarly | Plagino does not edit your writing |
| You want one report on a finished essay | Plagino | One PDF with sentence-level AI highlights and matched academic sources |
| You write formally or in a second language | Plagino | It has a published 0.8% false positive rate; Grammarly publishes none |
| You need a figure you can cite | Plagino | Its detection and false positive rates come from a fixed 1,000-document test |
Test conditions and limitations of this comparison
Plagino's figures come from one benchmark run in March 2026: 1,000 documents (500 AI-written, 500 human-written), each tool used through its own web app on default settings, scored on whether a document was flagged at all. The 95% intervals are Wilson intervals on 500 documents per rate. Results on your own writing will differ, and a different corpus could rank the tools differently. The full method is on our benchmark page.
Grammarly was not in that run, so this page compares a measured result with vendor claims and competitor tests. They are not like for like. Grammarly's 99% is its own claim, RAID uses a different corpus and metric, and every third-party test above came from a rival vendor with a sample of 99 texts or fewer. Prices and features were checked between 3 and 18 October 2026 and change often. Comparing other tools? See Plagino vs GPTZero, Plagino vs Scribbr, Plagino vs Copyleaks and our best AI detectors roundup.
Frequently asked questions
Is the Grammarly AI checker free?
Yes. The AI checker on Grammarly's website is free and returns a single percentage. The AI Detector agent inside Google Docs, Word and the Grammarly editor needs a paid plan such as Grammarly Pro.
Can my teacher see my Grammarly AI score?
Not through Grammarly. The free checker and the AI Detector agent show the score to whoever runs the check, and Grammarly describes no feature that sends it to an instructor. A teacher who wants a score has to run their own check, usually with a different tool such as Turnitin.
Can Grammarly's AI detector detect ChatGPT?
Grammarly says it detects text from ChatGPT, Gemini, Claude, Microsoft Copilot and other tools, but it publishes no detection rate for any model. In Pangram's small January 2026 test, the free checker did not flag any of three GPT-4o samples at a 75% threshold.
Why does Grammarly say my writing is AI-generated?
Your writing may share patterns, syntax or complexity with AI text. Grammarly's user guide also says that rewriting a draft with Grammarly or another AI tool raises the score, so heavy use of AI rewrite features can push your own work up.
Will using Grammarly get my essay flagged by Turnitin?
Spelling and grammar fixes are unlikely to be flagged. Turnitin has said that larger AI rephrasing across several sentences in a row likely will be, so Grammarly's generative and rewrite features carry the most risk. Check your school's AI policy before using them.
Does Grammarly's AI score match Turnitin?
No. Grammarly uses its own in-house model, so its score can differ from Turnitin, GPTZero or Plagino on the same text. A low Grammarly score does not predict a low Turnitin score.
Is Grammarly's AI detector better than GPTZero?
There is no like-for-like test to say. GPTZero was in Plagino's March 2026 benchmark, where it caught 88.9% of AI documents and wrongly flagged 5.7% of human ones, but Grammarly was not, and Grammarly publishes no comparable figures.
Did Grammarly buy GPTZero?
Grammarly's parent company, Superhuman, announced an agreement to acquire GPTZero on 23 June 2026. The announcement said GPTZero would keep running as a standalone product.
What is the difference between Grammarly's AI detector and Grammarly Authorship?
The AI detector estimates from finished text how much of it looks AI-generated. Authorship records how a document was written, sentence by sentence, and lets you share a replay. Authorship is opt-in and must be turned on before you start writing.
How much text does Grammarly's AI detector need?
Grammarly does not publish a minimum or maximum length. Its user guide says shorter passages are harder to score accurately, so a score on a single paragraph deserves less weight than one on a full essay.
Does Grammarly keep the text I check?
Grammarly says checked text will never be publicly searchable or stored in a database others can access. The Superhuman privacy policy says it does not sell user content and lets you control whether your content trains its AI models, but it states no fixed retention period.
Has Plagino tested Grammarly's AI detector?
Not yet. Grammarly was not part of Plagino's March 2026 benchmark, which covered Turnitin, Plagino, Originality.ai, Copyleaks, GPTZero, Academi.cx and TurnDetect. This page uses Grammarly's own claims and third-party tests instead, and labels which is which.
