Grammarly AI Detector Review: The Mainstream Brand That Refuses to Overclaim

Grammarly's AI detector is a free, competent screening tool wrapped in the most cautious marketing in the category — the company states plainly that no detector can "conclusively or definitively determine whether AI was used." Its percentage scores are useful as a first pass. Its genuinely interesting feature, though, is Authorship, which tracks how a document was written instead of guessing afterward.

That's the short version. The longer version involves a 99% accuracy claim sitting on the same page as a warning not to trust any accuracy claim, a provenance feature that sidesteps the entire statistical-detection debate, and a pricing structure where the detector is mostly a supporting act. All of it deserves a closer look.

One disclosure before we start: we build a detector and an AI humanizer ourselves at HumanFlow, which makes us a competitor writing about a competitor. Here's our editorial policy — judge accordingly. We fetched Grammarly's live pages while writing this review rather than working from memory, and every claim below is either quoted from those pages or marked for verification.

What Grammarly's AI detector actually is

Grammarly is a writing-assistance company first. Roughly 40 million people use it daily for grammar, tone, and clarity suggestions, and the AI detector arrived as one feature among many rather than as the product itself. That context matters, because it explains almost everything about how the tool is positioned.

The detector scans text for what Grammarly describes as "language patterns that are often linked to AI-generated writing" — content produced by ChatGPT, Gemini, Claude, Microsoft Copilot, and other models. You paste text in, and it returns a percentage representing how much of the document appears AI-generated, with results framed as "Resembles AI text" or "No AI text patterns found."

Notice the hedged verbs. Resembles. Patterns. Not "written by AI." That's not lazy copywriting; it's an accurate description of what statistical detection can and cannot do, and Grammarly is one of the few vendors that consistently phrases it this way. Detectors measure how machine-typical a text is — its predictability and rhythm — not who actually wrote it. If you want the mechanics, our explainer on how AI detectors work walks through perplexity and burstiness in detail. The one-sentence version: a detector is a probability engine judging whether your word choices look like the ones a language model would have made, which is also, uncomfortably, what plain competent prose sometimes looks like.

Free vs. Pro: what you get at each tier

The basic detector is free. You get the percentage score and the resembles/doesn't-resemble verdict, and for a lot of people that's the whole use case: paste, scan, done.

Grammarly Pro, at $12 per month at the time of writing (per Grammarly's plans page; a 7-day trial is offered), upgrades the detector into what Grammarly calls an "AI Detector agent." The paid version breaks down which sections triggered the flag, offers in-line rewrites, and generates citations. Pro also bundles plagiarism detection, unlimited writing suggestions, and the rest of Grammarly's paid feature set — which reinforces the point that the detector is a component of a suite, not a standalone product with its own pricing logic.

Pricing and tier details fetched from grammarly.com in August 2026; Grammarly changes plans periodically, so check the live page.

Compare that with the category's specialists. QuillBot's detector gives you 1,200 words per free scan with sentence-level analysis; Scribbr's free tool offers unlimited 1,200-word checks with no sign-up. Grammarly's free tier is thinner on detection detail than either. What it has that they don't is distribution: the detector lives inside an ecosystem already installed in millions of browsers, Word instances, and phones.

The 99% claim — and the fine print Grammarly itself supplies

Grammarly's detector page claims the tool "achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark." Two things about that claim, one generous and one skeptical.

The generous reading: RAID is a real, serious benchmark — an academic adversarial-robustness benchmark for AI detectors presented at ACL in 2024, and the closest thing this category has to an independent public standard. Citing it is meaningfully better than the free-floating "98.7% accurate!" numbers many detector vendors print with no methodology at all. We've written at length about how to read any detector's accuracy claim, and "names an external benchmark" clears a bar most of the market doesn't.

The skeptical reading: a benchmark score is a score on that benchmark's test set, under that benchmark's conditions. It is not a promise about your essay, your language background, or your professor's threshold settings. Published independent evaluations of the category find accuracy holding up on unedited model output and falling off hard on edited, mixed or paraphrased text — Perkins et al. measured 39.5% dropping to 22.2% — while false positives concentrate on non-native English prose, which Liang et al. measured at 61.22%. There's no reason to believe Grammarly escapes that pattern, and — to its considerable credit — Grammarly doesn't claim it does. We could not confirm the specific RAID leaderboard position independently, so treat "#1" as the vendor's characterization of a public leaderboard rather than as a finding of the paper — RAID's own four commercial detectors were GPTZero, Originality, Winston and ZeroGPT.

Note the oddity that both Grammarly and QuillBot cite RAID-based 99% figures on their respective pages. Both can be true under different RAID configurations and dates, which is precisely why a single benchmark number should never end your evaluation.

The cautious positioning — credit where it's due

Here is what Grammarly's own detector page says, verbatim: "No AI detector is 100% accurate." And: "Currently there is no AI detector that can conclusively or definitively determine whether AI was used." And that detection results should be "one part of a holistic approach to evaluating writing originality."

Sit with that for a second. This is a company that could sell certainty — the market rewards certainty; frightened students and suspicious editors pay for certainty — choosing instead to print, on its own sales page, that certainty is not available at any price. Grammarly also acknowledges that detectors "can indicate characteristics found in human-written text," which is a polite way of saying this tool will sometimes flag innocent people.

We're biased toward this framing because it's our framing too — HumanFlow refuses to publish an accuracy percentage for its own detector without published methodology, for exactly these reasons. But bias doesn't make the observation wrong. In a category where the OpenAI classifier was retired in July 2023 after catching only 26% of AI text while falsely flagging 9% of human writing (per OpenAI's own announcement), and where Liang et al. found seven detectors falsely flagging an average of 61.22% of human-written TOEFL essays (Patterns, 2023), a vendor that leads with humility is doing responsible engineering communication. Grammarly deserves the credit.

The cynic will say a brand with 40 million daily users simply has more to lose from an overclaiming scandal than a startup does. Probably true. The incentive and the ethics point the same direction here, which is the happiest case.

Authorship: the more interesting half of the story

If Grammarly's detector is a decent version of a flawed idea, Authorship is a different idea entirely — and, in our view, the more important one.

Authorship doesn't guess where your text came from. It watches. As you write in Google Docs (via the browser extension) or Microsoft Word (via the desktop app), it categorizes text by how it arrived: typed by you, generated by AI, pasted from a website, or edited through Grammarly's suggestions. When you're done, it produces a shareable report that color-codes the document by source, gives percentage breakdowns, offers pre-formatted citations in APA, MLA, and Chicago style, and — this is the striking part — includes a document replay showing "your writing process from the first paste to the last keystroke."

This is process provenance, not statistical guessing. A detector examines a finished artifact and estimates probabilities. Authorship records the construction of the artifact as it happens. A student who drafted an essay across three evenings, with visible typing, revision, and the occasional pasted quotation, has something no detector score can offer: evidence of how the work was made. For anyone facing the false-positive lottery — and non-native English speakers, heavy self-editors, and writers of formulaic academic prose face it most — that's a fundamentally stronger position than "the detector said 12%."

Current status, verified against Grammarly's Authorship page at review time: the feature exists in beta on desktop, is available on free and paid tiers, and Grammarly openly acknowledges "occasional inconsistent attribution" while it's refined. Beta features move, so treat the status as of our August 2026 review rather than as permanent. The honest caveats apply in both directions, too. Provenance tracking only works if it's running before you start writing; it can't retroactively vouch for last month's essay. It requires trusting Grammarly's telemetry. And a determined cheater can type AI output in by hand, so it's evidence, not proof. Still: as a way out of the detection arms race — for students who want protection rather than a verdict — it's the most interesting thing any mainstream writing company has shipped in this space.

What we have not done

We have not run our own hands-on test of Grammarly's detector. Everything above rests on vendor claims and published research, and that is exactly how you should weight it. We would rather say so than publish a table of numbers we did not measure — and it is why the next section hands you the method instead of asking you to trust ours.

Who should use it — and who shouldn't

Use Grammarly's detector if you're already a Grammarly user and want a quick screening pass — a student sanity-checking a draft before submission, an editor triaging freelance copy, a teacher who wants a second signal (never a first or only one). The price of entry is zero and the tool won't lie to you about what it knows.

Use Authorship — seriously, consider it — if you're a student writing in an environment where AI accusations happen. Starting your drafts with provenance tracking on is cheap insurance that exists before you ever need it.

Skip Grammarly's detector if you need sentence-level forensic detail on the free tier (QuillBot and Scribbr both give you more granularity for free), if you need bulk scanning or an API for editorial workflows (this isn't that product), or if you're an institution looking for something to base misconduct decisions on. On that last one, Grammarly's own page has already told you not to — and it's right. The base-rate arithmetic is unforgiving: even a detector with a 1% false-positive rate, screening 10,000 honest documents, flags roughly 100 innocent writers. No tool in this review, ours included, escapes that math.

Limits, stated plainly

The free tier's single percentage is a blunt instrument; you can't see why text was flagged without paying. Grammarly publishes no false-positive rate for the detector, and its accuracy claim inherits every limitation of benchmark-based claims. Language support for detection isn't specified on the product page at all, which for a tool used by non-native English writers is the omission that matters most, which matters because detector reliability on non-English and non-native-English text is the category's documented weak point — see our rundown of detector false positives for why that group bears the risk. Authorship is beta, desktop-bound, and only covers documents where it was running from the start. And like every statistical detector, the core tool will be least reliable exactly where the stakes are highest: short texts, edited texts, and honest writers with machine-typical prose styles.

None of these limits are hidden. That's the review in one line: an ordinary detector, extraordinarily honestly sold.

Where it sits among the alternatives

Against the specialists, Grammarly trades depth for reach and restraint. QuillBot's detector offers more generous free analysis and 20+ languages, with its own conflict-of-interest wrinkle — it sells a humanizer, too. Scribbr's detector pairs a genuinely free unlimited tool with the category's most transparent self-published research. Our full detector accuracy hub compares the field, and if you want a second opinion on any specific text, our own AI detector gives sentence-level readouts free up to 10,000 words a month — with the standing caveat that it doesn't promise to beat or out-judge any other tool, because nobody can honestly promise that.

FAQ

Is Grammarly's AI detector free? The basic detector is free: paste text, get a percentage score and a "Resembles AI text" or "No AI text patterns found" verdict. The detailed section-by-section breakdown, in-line rewrites, and citation generation require Grammarly Pro, listed at $12/month at review time.

How accurate is Grammarly's AI detector? Grammarly claims 99% detection accuracy citing the RAID benchmark, while stating on the same page that "no AI detector is 100% accurate." Benchmark scores describe performance on a fixed test set; real-world accuracy drops on edited, mixed, and non-native-English text. No published independent figure specific to Grammarly's false-positive rate was available at review time.

What is Grammarly Authorship and how is it different from AI detection? Authorship tracks how a document was actually written — typed, AI-generated, pasted, or Grammarly-edited — as you write in Google Docs or Word, then produces a color-coded report with a writing-process replay. It records provenance rather than estimating probabilities after the fact, so it can serve as affirmative evidence of your own work. It's in beta and only covers writing done while it's active.

Can Grammarly's detector prove I used ChatGPT? No, and Grammarly says so itself: no detector "can conclusively or definitively determine whether AI was used." A high score means your text statistically resembles machine output. Innocent explanations — formulaic structure, non-native phrasing, heavy editing — produce the same signal.

Which AI models does Grammarly detect? Grammarly says the tool scans for content generated by ChatGPT, Gemini, Claude, Microsoft Copilot, "and other AI models." As with all detectors, coverage of the newest model releases lags their launch, and detection of edited or paraphrased output is weaker than detection of raw output.

Should teachers use Grammarly's detector for misconduct decisions? As the only evidence, no — Grammarly itself frames results as "one part of a holistic approach." A score can justify a conversation, never a verdict. False-positive risk falls disproportionately on non-native speakers and rigid-structure writers, and a 1% error rate at institutional scale still means many innocent flags.

Is Grammarly's AI detector better than QuillBot's or Scribbr's? They're close enough that "better" depends on your use. Grammarly wins on ecosystem integration and cautious framing; QuillBot on free-tier depth and language count; Scribbr on published comparative research. All three carry the same structural limits of statistical detection.

Key facts

• Grammarly's basic AI detector is free; the detailed "AI Detector agent" ships with Grammarly Pro at $12/month (grammarly.com/plans, fetched August 2026).
• Grammarly claims "99% detection accuracy" and a #1 ranking on the RAID benchmark (ACL 2024) — a vendor claim citing an independent benchmark. QuillBot cites the same benchmark for a 99% claim of its own, which is the tell: RAID's own conclusion is that detectors advertising "extremely high accuracy (99% or more)" are "easily fooled by adversarial attacks".
• The same page states "no AI detector is 100% accurate" and that none can "conclusively or definitively" establish AI use (grammarly.com/ai-detector).
• Authorship tracks text provenance (typed / AI / pasted / edited) in Google Docs and Word, in beta at review time, on free and paid tiers (grammarly.com/authorship).
• OpenAI retired its own classifier in July 2023 after 26% true-positive and 9% false-positive performance (OpenAI announcement).
• Liang et al. (Patterns, 2023) found 61.22% average false-positive rates on human-written TOEFL essays across seven detectors — Grammarly was not among those tested.
• Detection targets include ChatGPT, Gemini, Claude, and Microsoft Copilot per Grammarly's product page.

Sources

1. Grammarly — AI Detector product page (grammarly.com/ai-detector), fetched August 2026.
2. Grammarly — Authorship product page (grammarly.com/authorship), fetched August 2026.
3. Grammarly — Plans and pricing (grammarly.com/plans), fetched August 2026.
4. Dugan, L., Hwang, A., Trhlík, F., Ludan, J.M., Zhu, A., Xu, H., Ippolito, D. & Callison-Burch, C. (2024). RAID: A Shared Benchmark for Robust Evaluation of Machine-Generated Text Detectors. ACL 2024. arXiv:2405.07940.
5. OpenAI — "New AI classifier for indicating AI-written text" and retirement update, July 2023.
6. Liang, W. et al., "GPT detectors are biased against non-native English writers," Patterns (Cell Press), 2023.