Grammarly AI Detector Review: How Accurate Is It Really?

Grammarly's AI detector is free and claims 99% accuracy on the RAID benchmark. Here is what that number really means for your writing.

Grammarly's AI detector is free, gives you a percentage of your document that "resembles AI text", and Grammarly claims 99% detection accuracy while citing a top ranking on the independent RAID benchmark. That headline number is real. It also describes a benchmark, not your essay.

The difference between those two things is the whole review.

Key Takeaways

• The basic detector is free. Grammarly Pro adds an "AI Detector agent" with deeper analysis, but the score itself does not sit behind the paywall.
• Grammarly claims 99% detection accuracy and says it ranks first on RAID, a shared academic benchmark for machine-generated text detectors.
• Detection accuracy and false-positive rate are different numbers. A tool can catch 99% of AI text and still flag human writing at a rate that matters to you.
• It outputs two buckets: a percentage that resembles AI text and a percentage with no AI patterns found, plus sentence-level highlighting.
• Paraphrasing is the known weak point for every detector in this class, Grammarly included.

What the tool actually does

Paste text in, and Grammarly segments it and analyses each section for language patterns associated with machine generation. You get back a percentage split into "resembles AI text" and "no AI patterns found", with the flagged passages highlighted so you can see which sentences drove the score.

That sentence-level view is genuinely the best part. A single document-level percentage tells you almost nothing actionable. Seeing that three paragraphs in your methodology section carried the entire score tells you something.

Grammarly is also unusually clear in its own messaging that no AI detector is 100% accurate and that detection results should not be used alone. That caveat is on their own product page. Credit where it is due, because plenty of competitors bury it.

About that 99% figure

Here is the part worth slowing down for, because it applies to every detector that quotes a headline accuracy number, ours included.

RAID is a real, serious benchmark. It was built by academic researchers as a shared evaluation set for machine-generated text detectors, specifically to test resilience against adversarial tricks like paraphrasing, synonym swapping and homoglyph substitution. Ranking well on it is a meaningful result, not marketing noise. You can read the RAID benchmark paper yourself.

But a benchmark score answers a specific question: on this curated set of documents, how often did the classifier assign the correct label? Your question is different. Yours is "will this tool flag the essay I actually wrote?"

Those come apart for two reasons.

Benchmark text is not your text. Evaluation sets are built from generated samples and human samples that are usually clean, full-length and monolingual. Your document might be a 400-word discussion post written by someone who learned English at nineteen. Detector error rates climb on both short text and non-native English writing, and benchmarks rarely stress that combination.

Detection accuracy is not the number that hurts you. If a tool catches 99% of AI text, that is the true-positive rate. The number that ruins your week is the false-positive rate: how often it flags human writing. A published detection-accuracy figure tells you nothing about it unless the false-positive rate is published alongside, on the same test set.

That is why our own 600-sample benchmark reports both numbers together across ChatGPT, Claude, Gemini, DeepSeek, Grok and LLaMA. A single accuracy figure without its companion is half an answer.

Where Grammarly's detector is genuinely useful

It is a good self-check tool, and it fits a specific workflow well.

If you are already writing inside Grammarly, the detection sits next to the grammar suggestions with no extra step. For a writer who wants a sanity check before sending a draft to a client or an instructor, that convenience is worth real money. The sentence highlighting makes it diagnostic rather than just judgmental.

It is also reasonable for content teams doing first-pass screening on freelance submissions, provided nobody treats the output as proof. We wrote about what that screening workflow looks like after auditing 50 freelance articles.

Where it falls down

Three limits matter.

Paraphrased AI text. This is the structural weakness of the entire detector category. Text generated by a model and then rewritten, by a human or by a paraphrasing tool, loses many of the statistical fingerprints these classifiers rely on. Grammarly is not uniquely bad here. Nothing in this class handles it reliably, which is why we cover whether AI detectors can catch paraphrased text separately.

Short passages. Below roughly 300 words, every detector's confidence degrades sharply, because there is simply not enough signal. Treat any score on a paragraph as noise.

The conflict-of-interest question. Grammarly sells writing assistance, including generative features. A detector built by a company whose product also generates text occupies an awkward position, and it is fair to ask how its own outputs score. That is not an accusation, it is a reason to check any important document against a second, independent tool.

The honest comparison

If you want a second opinion that does not share a vendor with your writing assistant, run the same text through our free AI content detector. You get professional-grade detection across every major model at 99.99% accuracy on text, on a free account with free credits on signup and up to 50,000 characters per scan.

Two detectors agreeing raises your confidence a lot. Two detectors disagreeing tells you the text sits in the ambiguous band where no percentage should drive a decision, which is itself the most useful thing you can learn.

For a direct head-to-head on the two tools students encounter most, see our comparison of GPTZero versus Grammarly.

The verdict

Grammarly's detector is free, well built, honest in its own caveats, and genuinely useful as a self-check. The 99% figure is a real benchmark result rather than an invented number, which already puts it ahead of much of this market.

Just do not read it as "99% chance this verdict about my essay is right". It means something narrower, and the gap between the two is where false accusations live.

Use it. Verify anything consequential with a second tool. And never let a percentage be the only evidence in a conversation about someone's work.

How to read any detector's accuracy claim

Grammarly's number is better sourced than most, which makes it a good worked example of how to read these claims generally.

Start with Grammarly's own detector page, where the 99% figure and the RAID ranking are stated. Then ask four questions of that claim, or of any competitor's.

What was measured? Detection accuracy is the rate of correctly identifying AI text. It is not the rate of correctly leaving human text alone. A vendor quoting only the first number has told you the flattering half.

On what text? Benchmarks use curated samples: clean, full-length, usually monolingual. If your documents are short, edited, or written by someone working in a second language, benchmark performance does not transfer. Liang et al., 2023 demonstrated exactly this failure mode, finding sharply elevated misclassification on non-native English writing.

Against which models? A detector calibrated on older output measures the wrong statistical profile on current models. Claude and Gemini output sits closer to the human boundary than GPT-3 era text did.

Is the test set public? RAID is, which is the reason a RAID ranking means something. Most vendor accuracy claims rest on internal sets nobody can inspect.

Grammarly clears three of those four hurdles, which puts it ahead of most of this market. What it does not publish, and what almost nobody publishes, is a false-positive rate measured on the same set as the accuracy figure.

Until that becomes standard, the only safe way to use any detector is as one input among several. Not because the tools are bad, but because a single probability presented without its error profile cannot carry the weight people keep putting on it.

FAQ

Is Grammarly's AI detector free?

Yes. The basic detector and its score are free. Grammarly Pro adds a deeper "AI Detector agent" with expanded analysis.

Is Grammarly's AI detector accurate?

Grammarly claims 99% detection accuracy citing the RAID benchmark. That measures how often it correctly identifies AI text on a curated test set, which is not the same as how often it falsely flags your human writing.

Does using Grammarly make my writing look AI-generated?

Grammar and spelling corrections do not meaningfully change the statistical patterns detectors read. Generative rewriting is a different matter, because that output is machine-generated text.

Can Grammarly detect Claude or Gemini?

It detects patterns associated with machine generation generally rather than identifying a specific model. No consumer detector reliably names which model produced a piece of text.

What should I do if Grammarly flags my original writing?

Check it against an independent detector, keep your draft history, and remember that formal, structured prose scores higher across every tool on the market.

Worth a second opinion

Grammarly's detector earns its place in a writing workflow. It should not be the only voice in the room when something important is on the line.

Run your text through GPTOne for a free second check. Free credits on signup, no card required.

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