Grammarly AI Detector Review: 99.5% on RAID, Users Report False Flags

Grammarly's AI detector is the free AI-content checker at grammarly.com/ai-detector, made by Grammarly, which is part of Superhuman, Inc. This review covers three separate things: the free detector scan, the AI Detector agent in Grammarly Pro, and Authorship, a record of how a document was written. It does not review Grammarly as a grammar checker.

What does Grammarly's AI detector percentage mean?

Grammarly's AI detector percentage is an estimate of how much of a text resembles AI-generated writing; it does not say who wrote the text. Grammarly's own FAQ says "No AI detector is 100% accurate" and that you "should never rely on the results of an AI detector alone."

Grammarly's FAQ, read on October 28, 2026, says the detector "segments the text and analyzes each section for language patterns that are often linked to AI-generated writing." The result screen splits the text into a share that "Resembles AI text" and a share where "No AI text patterns found." Formal, predictable prose can resemble AI text whoever wrote it.

What Grammarly AI score is acceptable?

Grammarly publishes no pass mark: its detector page, read October 28, 2026, gives no score below which a text counts as safe, and says "AI detection should never be used as a standalone verification method." What counts as a problem is set by your school's or client's own tool and policy, not by Grammarly. A low Grammarly reading also does not predict what Turnitin, GPTZero or Copyleaks will return on the same text.

What is Grammarly's AI detector?

Grammarly's AI detector is a free scan on the grammarly.com/ai-detector page that returns a percentage. Grammarly Pro adds the AI Detector agent, which, per Grammarly's FAQ, helps you "see exactly which parts may sound AI-generated." Authorship is a different tool: it records the writing process instead of judging the finished text.

The upload box accepts .doc, .docx, .odt, .rtf, .txt, .html and .pdf files. Grammarly says the detector identifies text from "ChatGPT, Gemini, Claude, Microsoft Copilot, and more." Grammarly sells AI writing tools on the same site, and its footer lists an AI Humanizer next to the AI Detector; the detector page also offers "one-click rewrites with AI Rewriter" for flagged phrases.

Does Grammarly keep the text you scan, and which languages does it check?

Grammarly's AI detector page does not say whether scanned text is stored or used to train its models; on October 28, 2026 the only privacy line on the page was a pointer to Grammarly's Terms and Privacy Policy, so read those before pasting unpublished or confidential work. The page also names no supported languages and gives no accuracy figure for any language other than English, so a result on non-English text rests on nothing Grammarly has documented.

Who owns Grammarly, and why does it matter for GPTZero?

Grammarly is part of Superhuman, Inc., per its own sign-in screen, and on October 28, 2026 Superhuman's website listed both Grammarly and GPTZero among its products. We read no announcement giving a date or terms, but a GPTZero-versus-Grammarly comparison is no longer one between unrelated companies.

How accurate is Grammarly's AI detector?

On the public RAID benchmark, Grammarly's detector caught 99.5% of AI-generated texts when held to a 5% false-positive rate and 98.2% when held to 1%, with an AUROC of 99.9%, per its entry on the RAID leaderboard dated February 9, 2026. Those figures include every attack in RAID's test set, meaning AI text deliberately altered to slip past detectors; without those attacks the scores were 99.9% and 99.7%.

RAID is a benchmark built by researchers (Dugan et al.), and its leaderboard scores any detector that submits results. Grammarly's own page says the detector "achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark." The 99% matches the leaderboard entry; we did not check the #1 position against the other submissions, so that part stays Grammarly's claim. The same page says the detector is "Designed to avoid wrongly flagging human-written text," with no figure attached.

What does the RAID result measure, and what does it leave out?

The RAID result measures how many AI-written texts Grammarly's detector catches on RAID's own test set; it does not tell you how often your human writing will be flagged on Grammarly's website. RAID's "accuracy" here is the share of AI-generated texts caught, at a false-positive rate RAID fixes for each domain. The free web scan uses its own threshold, which Grammarly does not publish, so the benchmark's 5% or 1% is not the rate a student faces.

Grammarly ran its detector on RAID's test texts and submitted the predictions, and RAID scored them against labels it keeps hidden. The scoring is RAID's; the run is Grammarly's, and a leaderboard entry cannot show that the submitted model is the one the website runs today. RAID's texts also come from its own fixed set of generators, which predates many of the current ChatGPT, Claude and Gemini models Grammarly says it detects.

Grammarly's FAQ adds one more detail: its model "was trained on tens of thousands of texts, including both human-written and AI-generated text created before 2021." Most models Grammarly says it detects came out after 2021, and beyond a line that the detector "Updates continuously," the page does not say whether newer training data was added.

Which accuracy figures come from someone other than Grammarly?

Two outside sources score Grammarly's detector, and they disagree sharply: RAID's leaderboard, which scored predictions Grammarly submitted, and a November 2025 test by Originality.ai, a company that sells a competing detector, which reported a recall of 0.222. The table puts each figure next to its source and that source's interest.

The Originality.ai page ranks its own detector first (F1 0.918), tests no human-written text for false positives, and reports a Grammarly "accuracy" of 22.22% that is identical to the recall of 0.222, which suggests the two measures were mixed up. It also predates Grammarly's February 2026 leaderboard entry by three months, so the two may not describe the same version of the detector. Other figures in circulation, from Quetext, from fast.io and credited by fast.io to Pangram Labs, rest on a single sample, a competing seller or a secondhand report, so they are left out.

Does Grammarly's AI detector work on edited or paraphrased text?

Partly, on the evidence available: across RAID's full test set, which includes AI text deliberately altered to evade detectors, Grammarly's catch rate at a 5% false-positive rate was 99.5%, against 99.9% with no alterations. Those are RAID's own alterations, and no figure has been published for AI text a person has rewritten by hand. One informal report points to a gap there: a commenter on r/WGU wrote that it "misses ai content that's been lightly edited" (September 15, 2026).

Does Grammarly's AI detector flag human writing?

Yes, users report it: about half of the twelve dated Reddit comments cited on this page describe human writing scored as AI, at readings from 30% to 81%. Grammarly says its model is "designed to minimize false positives" but publishes no false-positive rate for the web scan, and the RAID result holds the false-positive rate fixed by design, so there is no published number to set these reports against. None of the reports can be checked against who really wrote the text.

The category-wide evidence is older: Liang et al. (Patterns, July 2023) found seven detectors averaged a 61.3% false-positive rate on 91 TOEFL essays by non-native writers. Grammarly's detector was not among them.

Is Grammarly's AI detector free?

Yes: the scan on Grammarly's AI detector page is free and returns a percentage without a paid plan. The page states no length limit; its text box accepts up to 25,000 characters, and longer work can be uploaded as a file. What the free scan does not give you is a sentence-by-sentence view, which Grammarly reserves for the AI Detector agent in Pro.

How do you use Grammarly's AI detector?

Grammarly's AI detector takes three steps, per its own page: paste your text or upload a document into the box at the top of grammarly.com/ai-detector, click "Scan for AI," and read the percentage of the text that "appears to be AI-generated." A "Try sample text" button shows a demo result first. The free result is that percentage and the split between text that resembles AI and text with no AI patterns found; which sentences drove it needs Pro.

How much does Grammarly Pro cost for AI detection?

Grammarly Pro costs $12 a member a month billed annually ($144 a year), or $30 billed monthly, as shown in US dollars on Grammarly's Pro page on October 28, 2026. The annual price is 60% below the monthly one, which matches the "Save up to 60%" label on the toggle. Pro is a full writing plan; there is no detector-only tier.

Grammarly's plans page showed Pro only as "$12 USD" with a free-trial offer and no billing period, so both toggles were read on the Pro page. Neither page shows a student price.

When does the free Grammarly AI detector stop being enough?

The free Grammarly scan stops being enough the moment a percentage needs explaining. A free reading tells you that, say, 30% of a text resembles AI writing, but not which sentences; to see which parts drove it, Grammarly sends you to the AI Detector agent in Pro, at $30 for a single month or $144 for a year. A commenter on r/CollegeRant who got "about 30% AI" clicked for an explanation and got "a 'weak signal'" (September 19, 2026).

Paying does not fix the deeper limit: sentence-level detail shows where the model sees AI patterns, not whether it is right about your text, and no accuracy figure specific to the Pro agent is published. Proof that you wrote something comes from Authorship, which is free; the annual plan only makes sense if you want Grammarly's writing features anyway.

What is Grammarly Authorship?

Grammarly Authorship is a free, opt-in feature that records how a document is written and sorts its text by source: typed, pasted, AI-generated, from a website or from a Grammarly suggestion. It produces a shareable report that colour-codes each sentence and can replay the document "from the first word to the last punctuation mark," per Grammarly's Authorship page, read October 28, 2026.

Authorship works in Google Docs (through the Grammarly browser extension), Microsoft Word (through the desktop app), the Grammarly Editor and Superhuman Go; the landing page also names Canvas LMS submissions. Anyone with a Grammarly account can turn it on, and Grammarly says it "is available in Grammarly Free and Grammarly Pro."

How do you turn on Grammarly Authorship in Google Docs or Word?

Grammarly Authorship is switched on through the fingerprint icon: install the Grammarly browser extension for Google Docs or the Grammarly desktop app for Microsoft Word, open the document, and enable Authorship from that icon before you start writing. Grammarly's Authorship page gives these steps for Google Docs (with a video) and says only "install the desktop app" for Word, with no written Word walkthrough.

How is Authorship different from an AI detector?

Authorship records the writing process; an AI detector infers from the finished text. Grammarly's own FAQ puts it this way: detectors "don't keep track of document history; they just analyze your text in a moment in time," while Authorship keeps track of "version history and where text in your writing came from." For a student facing a flag, a replay of the essay being drafted over several sessions answers a question no percentage can.

What can an Authorship report not show?

An Authorship report covers only what was written in a supported editor with Authorship on: a draft pasted in from another app shows as pasted text, and work done before it was enabled has no record. Grammarly says the report is private until you create it, make its link shareable and send it to someone, and that it "doesn't auto-send it to your professor."

On data, Grammarly's FAQ says Authorship is opt-in and stores "only data related to text you paste into Google Docs." Authorship drew little user discussion in the 2026 Reddit comments we gathered: apart from Grammarly's support account recommending it, it came up in one passing mention, so there is no user record yet of how a report holds up when a teacher reads it.

Does Grammarly's own editing make text read as AI?

Grammar and spelling fixes from Grammarly usually do not trigger AI detection, but text from its generative features can, according to Turnitin. Turnitin's AI detection FAQ says that "in most cases, changes made by Grammarly (free & premium) and/or other grammar-checking tools were not flagged," and that content from "draft generation, paraphrasing, summarizing, and other features" will "likely be flagged as AI-generated by our detector." This is about Grammarly's editor, not its detector.

A commenter on r/WGU wrote that "Grammarly sucks at detecting anything and makes anything it touches look more like AI" (August 31, 2026), and one on r/royalroad said even light AI involvement is "enough to get pinged by the detector (which grammarly often does)" (September 11, 2026). For how Turnitin builds its own score, see our explainer on Turnitin's AI indicator.

What do real users report about Grammarly's AI detector?

Grammarly's AI detector users on Reddit mostly report distrust: of the twelve dated comments cited here, from August 31 to October 7, 2026, nine are negative, two mixed and one positive. About six describe human writing scored as AI and about four describe results that swing; some fit more than one group. Twelve comments show how the tool is experienced; they cannot show how often it is wrong.

On human writing scored as AI, a commenter on r/NoStupidQuestions said a term paper written by someone they had watched work on it was "flagged as 81% AI-generated, which was completely baffling because I have seen her laboriously writing that paper" (September 28, 2026). On r/selfpublish, a novelist wrote that "Grammarly ai detector said my novel was 80% ai then i put the book of Don Quixote de La mancha through it" (September 11, 2026). The r/CollegeRant commenter quoted above, who describes themselves as an academic, asked after a 30% reading: "I am an academic. Am I now AI?"

On unstable or untrusted results, a commenter on r/Grammarly reported that a passage "initially was not flagged at all on Grammarly, but I changed the em-dash to a semicolon, and it flagged it as AI-assisted" (September 12, 2026). Another wrote "Grammarly kept flagging stuff I wrote myself, so I stopped trusting the percentage" (September 28, 2026). On r/Professors, a commenter claimed that a random paragraph dropped "into Grammarly, or some other AI detector" will come back with "50-70% of it" resembling AI writing patterns (September 13, 2026).

On its place in a student's workflow, a commenter on r/WGU urged others "to NOT use the grammarly AI detector," adding that the university "only uses grammarly for...grammar" (September 6, 2026). The r/WGU commenter quoted above on edited text also said it flags "clean, formal writing all the time" (September 15, 2026). On r/AccusedOfUsingAI, one user said they run it on their own work because "It will tell you exactly what sounds like ai" (October 1, 2026). The one positive outcome came from r/TurnitinScan, where a writer's human-written document scored "2% and 11%" on plagiarism and AI scans during a free trial (October 7, 2026).

Where do Grammarly's claims and user reports disagree?

Grammarly's claims and user reports disagree on false flags. Grammarly's detector page says the tool is "Designed to avoid wrongly flagging human-written text," and Grammarly's support account told users on r/Grammarly that "Our AI detection model is designed to minimize false positives" (reply one, reply two, late August and early September 2026). Against that, the r/NoStupidQuestions, r/selfpublish and r/CollegeRant reports above describe human writing at 30% to 81% AI, and the second support reply sits in a thread whose title reports an 83% reading. The strong RAID result does not settle this disagreement, because RAID measures how much AI text is caught at a fixed false-positive rate, not how the web scan treats a given human essay.

The support account conceded part of the point: it said results are "best viewed in a broader context rather than as an objective source of truth," acknowledged that AI text may go undetected, and pointed users to Authorship.

What has changed with Grammarly's AI detector?

The main documented change is Grammarly's entry on the RAID public leaderboard on February 9, 2026, which put a benchmark-scored accuracy figure on the record; by October 2026 Grammarly also sat beside GPTZero among Superhuman's products. The detector has no public changelog and no pricing history was retrieved, so this log lists only dated, sourced events.

• November 12, 2025: Originality.ai, also a competing seller, publishes its RAID-based Grammarly review (F1 0.364).
• February 9, 2026: Grammarly's detector is entered on the RAID leaderboard, which scores it at 99.5% of AI texts caught at a 5% false-positive rate and 98.2% at 1%, all attacks included.
• August 28 to September 1, 2026: Grammarly's support account replies on r/Grammarly that the model is "designed to minimize false positives" and points users to Authorship.
• August 31 to October 7, 2026: the twelve Reddit comments cited above are posted.
• October 25, 2026: Turnitin's AI detection FAQ, which covers Grammarly's editing features, is last updated.
• October 28, 2026: our read of Grammarly's pages finds Pro at $12 a month billed annually or $30 monthly, Authorship in the free plan, the "created before 2021" training note, and GPTZero listed beside Grammarly among Superhuman's products.

Grammarly AI detector vs Turnitin, GPTZero and Copyleaks

Grammarly's AI detector is a self-check tool for writers, while Turnitin is what many schools run, and a Grammarly result does not predict a Turnitin, GPTZero or Copyleaks result, since each uses its own model and threshold.

Grammarly vs Turnitin

Turnitin is licensed to institutions, so a student cannot run it on their own work, and a low Grammarly score says nothing certain about the report a teacher will see; how Turnitin's AI detection works covers its scoring.

Grammarly vs GPTZero

GPTZero has a standing free plan of 5,000 characters per scan and 10,000 words a month, per our GPTZero review. Its Writing Replay, the closest match to Authorship, comes only with the Premium plan at $23.99 a month, while Grammarly includes Authorship in its free plan. Both products now sit under Superhuman, per Superhuman's website on October 28, 2026, which matters when either company's marketing ranks itself above the other.

Grammarly vs Copyleaks

Copyleaks offers a logged-out scan of up to 25,000 characters and paid AI and plagiarism checking from $13.99 a month billed annually (October 26, 2026, per our Copyleaks review); schools choosing a campus-wide tool can start with our Copyleaks vs Turnitin comparison.

If you want other options, our Originality.ai review covers a paid, credit-based detector, and our ranking of AI content detectors puts the rest side by side.

What should you do if Grammarly flags your own writing?

If Grammarly flags writing you did yourself, read the percentage as a description of style and collect evidence of how you wrote it: version history in Google Docs or Word, earlier drafts, notes and sources. Grammarly's own FAQ says a detector result should never be relied on alone.

For future work, switch on Authorship before you start, since it records only what happens after it is enabled, and ask which tool your school actually uses. If you teach or edit, ask for process evidence before acting on any score; Grammarly's Authorship FAQ suggests using the report to open a conversation, not to penalise.

Is Grammarly's AI detector worth using?

Grammarly's AI detector is worth using as a free first look at how a text reads, and on the public RAID benchmark it caught 99.5% of AI-generated texts at a 5% false-positive rate. Authorship is worth switching on for anyone who may one day need to show how they wrote something. Neither is worth paying for on its own account, and no one should treat the percentage as proof. We rate Grammarly's AI detector 3.6 out of 5.

Against it, the RAID figure was measured on RAID's older generators at a false-positive rate the web scan does not use, Grammarly publishes no false-positive rate for the scan itself, and 2026 user reports describe human writing scored as AI. Pay $30 for a month of Pro only if you need sentence-level detail on one document. Every review we publish is listed in our AI tool reviews index.

How did we review Grammarly's AI detector?

Grammarly's AI detector was reviewed from the public record, not from a test of our own. No account was created, no text was scanned, and no figure on this page is our measurement; the 60% annual saving and the $144 yearly cost are our arithmetic from Grammarly's published prices, and the RAID percentages are rounded from the leaderboard's own results file.

Grammarly's detector, Authorship, plans and Pro pages were read on October 28, 2026, on both billing toggles in US dollars, as were Superhuman's and GPTZero's homepages and Grammarly's RAID leaderboard submission. User experience comes from twelve dated Reddit comments and two Grammarly support replies, each linked; G2, Trustpilot and Capterra were not read. Our review methodology and source list set the standard every review follows, and we check this page monthly and update it when Grammarly changes its prices or publishes new accuracy figures.

Does AI Tools Police earn a commission on Grammarly?

This page carries no affiliate link to Grammarly, and AI Tools Police earns nothing if you sign up. We sell no detector and no humanizer. Originality.ai and Quetext, mentioned above, sell competing detectors, which is why each third-party figure names its source's commercial interest. Our affiliate standards set out how commissions, where they exist, are kept apart from verdicts.