Is Grammarly's AI detector accurate?
Grammarly publishes no accuracy figure and states plainly that its AI detection "is not 100% accurate and should not be used as a definitive assessment." It splits text into sections, scores each for AI-like patterns, and is deliberately tuned to minimize false positives. Grammarly's separate Authorship feature — which records typing versus pasting — is far stronger evidence.
Why — the first-principles explanation
Grammarly ships two different things, and the accuracy answer is opposite for each.
The AI Detector works like every other statistical checker: it breaks your text into smaller sections and checks each against a model for "language patterns, syntax, and complexity," then reports the percentage of scanned text that looks likely AI-generated. That's inference from style, and it inherits every weakness of the method. Grammarly says so in its own documentation, unusually bluntly: the result "should not be used as an objective source of truth, as AI detection of any kind can be prone to errors," and shorter passages are harder to score than longer ones. No accuracy percentage is published anywhere in the support docs.
Grammarly does state a design choice that matters: the model is optimized to minimize false positives, because it considers wrongly calling human text AI-generated more damaging than missing AI text. That's the same trade Turnitin made, and it has the same price — a detector tuned not to accuse the innocent necessarily misses a chunk of the guilty. It also means a low score is very weak evidence of anything, since the tool is biased toward saying "human" in the first place. That matches the independent finding across 14 detection systems: as a category they skew toward classifying output as human-written.
Authorship is a different animal entirely. It doesn't analyze finished text — it watches the writing happen, tagging each chunk as typed by hand, pasted in, or accepted from an AI suggestion, and produces a report of where the text came from. That's a record, not a guess. A paste event either happened or it didn't. This sidesteps the bias problem completely: Authorship has no opinion about your vocabulary, so it can't punish you for writing in plain English as a second-language speaker — the failure mode that made seven detectors flag about 61% of non-native TOEFL essays.
Authorship has its own hard limit, though: it only knows what happened inside Grammarly's field of view. It runs in Google Docs via the browser extension and in Microsoft Word. Write somewhere else, or retype AI text by hand, and it sees nothing.
An example that makes it click
Two security guards at a museum. The first stands at the exit and studies your face — nervous? avoiding eye contact? He'll accuse some honest tourists and wave through some calm thieves, because he's judging a vibe.
The second guard just watches the cameras. She doesn't care how you look. She can say: at 2:14 you walked in with an empty bag, at 2:31 the bag was full. That's not a judgment, it's a log. Grammarly's AI Detector is the first guard. Authorship is the second. And notice the second guard's real limit — she can only see the rooms with cameras. Write your essay in a text editor Grammarly isn't watching, and there's no footage at all.
Key facts
- Grammarly publishes no accuracy percentage for its AI Detector and states it 'is not 100% accurate and should not be used as a definitive assessment.'
- Grammarly's documentation states the percentage result 'should not be used as an objective source of truth, as AI detection of any kind can be prone to errors.'
- The AI Detector breaks text into smaller sections and checks each against a model for language patterns, syntax, and complexity, returning the share of scanned text that appears AI-generated.
- Grammarly states its detection model is 'optimized to minimize false positives,' because incorrectly identifying human text as AI is considered more detrimental than missing AI text, and notes shorter passages are harder to score accurately than longer ones.
- Grammarly Authorship is a separate feature that tags text as typed, pasted, or AI-suggested during writing; as of April 2025 it works in Google Docs via the browser extension and in Microsoft Word on Windows and Mac.
- Statistical detectors as a category skew toward classifying text as human-written (peer-reviewed test of 14 systems, December 2023), so low scores carry little information.
▶ The 60-second explainer (script)
Is Grammarly's AI detector accurate? Grammarly won't say — and to their credit, they're upfront about why. Their own support documentation states the detection is, quote, not one hundred percent accurate and should not be used as a definitive assessment. They also say the percentage should not be treated as an objective source of truth. There's no accuracy figure published anywhere. Here's how it works. It chops your text into sections and checks each one for AI-like language patterns, syntax, and complexity, then reports what share looks machine-written. That's guessing from style — same method as every other checker, same weaknesses. Grammarly does make one deliberate choice worth knowing: the model is tuned to minimize false positives, because they'd rather miss AI text than wrongly accuse a human. Good instinct. But it means a low score is nearly meaningless — the tool leans toward saying 'human' by default. Now, the thing Grammarly has that's genuinely different: Authorship. It doesn't analyze your finished essay. It watches you write, and tags every chunk as typed, pasted, or accepted from an AI suggestion. That's a log, not an opinion. A paste either happened or it didn't. And it can't punish you for writing plainly — which matters, because regular detectors flag about sixty-one percent of essays by non-native English speakers. The catch? Authorship only sees what happens in Google Docs and Word. Write anywhere else and there's no footage.
What authoritative sources say
People also ask
Is Grammarly's detector as good as Turnitin's?
Impossible to say — Grammarly publishes no accuracy, recall, or false positive figures, while Turnitin publishes testing across 800,000 pre-GPT papers. Grammarly is the more transparent about its limits and the less transparent about its numbers.
Does using Grammarly make my writing look AI-generated?
Grammar and spelling fixes are unlikely to move a score much. Heavy use of generative rewriting is different — that text was produced by a model and can score as such. Authorship tags AI suggestions separately for exactly this reason.
Can teachers see my Grammarly Authorship report?
Only if you share it. Authorship generates a report on your document showing where text came from; it's designed as something a writer can show a reviewer, not a surveillance feed.
Does Authorship catch AI text I retyped by hand?
No. It records typing, pasting, and accepted AI suggestions. Text you retype manually registers as typed, because that's literally what happened.
The same question, asked other ways
- Is Grammarly's AI checker accurate?720/mo