Grammarly AI Detector: Review & Free Check
Paste any text below for a free AI score, no account needed. This is Humanit's checker, not Grammarly's own. Under it: what Grammarly's percentage actually measures, how far to trust it, and what its June 2026 purchase of GPTZero changes.
Grammarly's AI detector is a free tool that estimates what percentage of your text appears AI-generated. It is separate from Grammarly Authorship, which records how text entered a document while you wrote it. Since June 2026, Grammarly's parent company, Superhuman, also owns GPTZero.
Works with every major AI model
How it works
- 1Paste your text. Drop in the writing you want to check against Grammarly-style detection.
- 2Run the free detector. Humanit returns a 0–100 AI-likelihood score, a verdict, and the signals behind it.
- 3See what reads as AI. The subscores show which patterns — predictable phrasing, uniform rhythm, stock vocabulary — are pulling the score up.
- 4Fix it if needed. If it reads as AI, send it to the humanizer to rewrite those signals, then re-check.
Two different features, constantly confused
Almost every question about "the Grammarly AI detector" is really about one of two separate things. The first is a conventional AI detector: it reads finished text and estimates a percentage of it that looks machine-generated, using the same statistical signals every other detector uses. The second is Authorship, which is not a detector at all. Authorship works inside the Grammarly editor and records how text arrived — typed by hand, pasted from elsewhere, generated by an AI prompt, or edited with Grammarly's own suggestions — then produces a report on the finished document. One guesses backwards from the words. The other watched it happen.
Why that distinction matters more than the score
It changes what is even possible. A statistical detector can be wrong in both directions and can be affected by rewriting, because it only ever sees the final text — that is the whole basis on which humanizers work. Authorship is a provenance record, and rewriting a document afterwards does not change what was logged while it was being written. If your institution asks for an Authorship report, no rewriting tool — this one included — changes what that report says, and any product implying otherwise is selling you something that does not work. We would rather tell you that than take the sale.
How Grammarly's AI detector scores text
Grammarly's detector is a free web tool: paste text, run the scan, and it reports one headline figure, in its own wording the "% of this text [that] appears to be AI-generated". Notice the convention. That is a share of the text, the way Turnitin reports, not the whole-document probability GPTZero gives, so a 40% from Grammarly and a 40% from GPTZero are not the same claim. Underneath, it scores what every statistical detector scores: how predictable the word choices are and how uniform the sentence rhythm is. The free scan sorts your text into passages that resemble AI writing and passages that do not; seeing exactly which phrases might get flagged, with one-click in-line rewrites, is part of Grammarly's paid plans.
Grammarly now owns GPTZero: what changed in June 2026
On June 23, 2026, Superhuman — the name Grammarly has operated under since it bought the Superhuman email app and rebranded — announced it was acquiring GPTZero. The price was not disclosed; GPTZero's co-founder put the startup at 19 million registered users and $30 million in annual recurring revenue. Superhuman's own line was that "two AI detectors are better than one", and as of September 2026 that is literally the arrangement: Grammarly's detector and the GPTZero AI detector still run as separate products with different scoring conventions, and no merged model has been announced. The practical takeaway is that a Grammarly score and a GPTZero score are no longer opinions from rival companies — but they are still two different measurements, and they will not always agree. Grammarly also sells an AI humanizer, which it says is not meant for bypassing detectors; our hands-on test of Grammarly's AI humanizer shows how its output scored.
Does Grammarly itself make your writing look AI-generated?
This is the anxious question underneath most searches on this topic, and it deserves a straight answer. Accepting a lot of Grammarly suggestions does tend to push writing toward the smooth, conventional, low-variance register that detectors score as machine-like — not because Grammarly wrote it, but because heavy grammar correction and AI generation both flatten the same irregularities out of prose. Grammarly's own Authorship reporting distinguishes "edited with Grammarly" from "generated by AI" for exactly this reason. If you are worried, the fix is not to stop proofreading: it is to leave some of your own rhythm alone, and to keep sentences varied in length even where a suggestion offers to tidy them.
How accurate is Grammarly's AI detector?
Grammarly's page advertises near-perfect detection accuracy and a #1 ranking on RAID, an academic benchmark for AI-text detectors. Hold two things alongside that. It is the vendor's own number on the vendor's chosen setup — GPTZero's site cites its own RAID results too, which tells you how much room one benchmark leaves for everyone to win. And Grammarly's page itself says no AI detector is 100% accurate and that detection should not be used as a standalone verification method. The independent record for the category is sobering: a 2023 peer-reviewed study in Patterns found more than 61% of genuine TOEFL essays by non-native English speakers misclassified as AI, and OpenAI withdrew its own AI-text classifier in July 2023 after, by its published numbers, catching 26% of AI text while wrongly flagging 9% of human writing. Authorship is a different kind of claim: a log of what happened in one editor, dependable about what it covers and blind to anything written elsewhere.
Check your text free, no account needed
The detector at the top of this page is free and needs no account for checks up to 300 words, as many times as you like; a free account checks up to 1,500 words at a time. It returns a 0–100 AI-likelihood score broken into sub-signals — perplexity, burstiness, AI vocabulary, discourse markers, structure, personal voice — and names the exact phrases reading most machine-written, so you know what to change rather than just that something is wrong. If a passage reads as AI, you can hand it straight to the humanizer and re-check the result here. Like Grammarly's score, it is an estimate — treat it as one.
Who it's for
| Feature | Grammarly AI detector | Humanit detector |
|---|---|---|
| Free, no account | Yes, basic scan | Yes, up to 300 words per check |
| What the score means | Share of the text that appears AI-generated | 0–100 likelihood for the whole text |
| Shows which phrases read as AI | On paid plans | Yes, free |
| Breakdown of why | Overall % plus AI-like passages | Sub-signals: perplexity, burstiness, structure and more |
| Rewrite flagged text | Paid in-line rewrites, plus a separate AI Humanizer | Humanizer on the same page |
| Tracks how text was written | Yes, via Authorship | No, it reads the finished text only |
| AI detector | Authorship | |
|---|---|---|
| What it does | Estimates an AI % from the finished text | Records how text entered the document |
| What it sees | Only the words you ended up with | Typing, pasting, AI generation, edits |
| Can it be wrong? | Yes — false positives and negatives | It reports what it logged, in that editor |
| Affected by rewriting after the fact? | Yes | No |
| Covers text written elsewhere? | Yes — any text you paste in | Only writing done inside Grammarly |
| Grammarly AI detector | GPTZero | |
|---|---|---|
| Owner | Superhuman (formerly Grammarly) | Superhuman, since June 2026 |
| Free without an account | Yes, basic scan | Yes, up to 10,000 characters per scan |
| What the % means | Share of the text that appears AI-generated | Likelihood the document is AI-written |
| Result labels | One overall percentage | Human, AI + Human, Polished by AI, Paraphrased by AI, or a model name |
| Process evidence | Authorship (writing done in Grammarly) | Writing Replay |
| Vendor accuracy claim | Near-perfect, "#1 on RAID" | Near-perfect, with its own RAID figures |
Frequently asked questions
Is this the official Grammarly AI detector?
No. This is Humanit's free detector for the same job — paste text, get an AI-likelihood score — so you can check before Grammarly or anyone else does. Grammarly's own detector and Authorship live inside Grammarly. We do not access your Grammarly account.
Is Grammarly's AI detector free?
Yes. Grammarly's web detector runs a basic scan free, without signing up, and reports an overall percentage. Phrase-level detail and one-click rewrites of flagged text sit on paid plans. The checker on this page is free with no account for up to 300 words per check.
Does Grammarly own GPTZero?
Yes, as of June 2026. Superhuman, the company behind Grammarly, announced on June 23, 2026 that it was acquiring GPTZero. As of September 2026 the two detectors still run as separate products with different scoring conventions.
Grammarly AI detector vs GPTZero: which is more accurate?
Both vendors advertise near-perfect accuracy and both cite the RAID benchmark, so the marketing will not settle it. They also measure different things: Grammarly reports the share of text that looks AI-generated, GPTZero the likelihood that the document as a whole is. Run the same text through both and treat disagreement as a sign the text sits in the grey zone, not as proof either way.
Does Grammarly detect AI writing?
Yes, in two different ways. Its AI detector estimates a percentage from the finished text, like any other detector. Separately, Authorship records how text entered the document while you were writing — typed, pasted, or AI-generated — and reports on that. They answer different questions and only the first is a guess.
What is the difference between Grammarly's AI detector and Authorship?
The detector reads the final text and infers. Authorship watched the document being written and logged it. That means rewriting a document can change what a detector says about it, but does not change what an Authorship report recorded.
Can a humanizer beat Grammarly Authorship?
No — and be sceptical of anything that says otherwise. Authorship is a record of how the text was produced, not an analysis of the words, so editing the words afterwards does not alter it. Humanizers work on statistical detectors, which is a different problem.
Does using Grammarly make my writing get flagged as AI?
Grammarly does not write your text, but accepting a lot of corrections smooths out the sentence-length variation and irregular phrasing that detectors read as human. That can nudge a score upward. Keeping some variety in your sentence lengths is the practical countermeasure.
How accurate is Grammarly's AI detector?
Grammarly advertises near-perfect detection accuracy; that is a vendor claim, and its own page adds that no detector is 100% accurate. Independently, detectors as a class show real false-positive rates — a 2023 study in Patterns found over 61% of genuine non-native English essays misclassified — and much weaker performance on edited AI text. Use any single score as a prompt to look closer, not as a conclusion.
Will my text be used to train AI?
No. Your text is sent to the model only to produce your result and is never used to train any AI. No content is stored beyond the request.
Does it keep my original meaning?
Yes. Humanit preserves your facts, numbers, names, and citations while changing the phrasing and structure. It never invents new claims.