Grammarly AI Detector: Free Tool, Accuracy and Limits (2026)

Quick answer: what the Grammarly AI detector is and where to find it

Grammarly offers AI detection in two forms. The first is a free AI detector on Grammarly’s website: paste text or upload a file, click Scan for AI, and it returns the percentage of the text that appears to be AI-generated. Grammarly states that no sign-up is required for this version. This is the tool most people searching for “AI detector Grammarly” are looking for.

The second is the AI Detector agent inside Grammarly’s paid plans. According to Grammarly’s support guide, it is available in Grammarly docs and the Superhuman Go assistant for Pro and Plus users, through the browser extension inside Google Docs, and through the Mac and Windows apps inside Microsoft Word via the plagiarism checker. Business, Enterprise and Education accounts get it in docs when agents are enabled by an admin. The paid agent goes further than the free tool: it marks the specific passages it thinks are AI-written, offers to generate a citation for AI use, and connects to Grammarly’s rewrite features.

This article is an independent explainer. We are not affiliated with Grammarly, and the links above go to Grammarly’s official pages.

How the Grammarly AI detector works

Grammarly describes the method in its support documentation. The detector splits your text into smaller sections and checks each one against a machine learning model trained on what Grammarly says are hundreds of thousands of human-written and AI-generated texts. It looks for language patterns, syntax and complexity that are typical of AI output, then averages the results into one percentage for the whole document. For a general explanation of the techniques behind tools like this, see our guide on how AI detectors work.

Two details from Grammarly’s own documentation are worth knowing before you read a score. First, the tool does not explain why a passage was flagged; it reports a likelihood, not a reason. Second, the percentage is described by Grammarly as an averaged estimate across the document, not a sentence-by-sentence verdict. The paid agent narrows that down by marking sections, but the free web tool gives you the single number only.

How accurate is it? Grammarly’s claims versus what is independently known

On its product page, Grammarly says the detector achieves 99% detection accuracy and ranks #1 on RAID, an independent benchmark that compares AI detectors on a large shared dataset. Grammarly’s February 2026 announcement describes RAID as testing more than 670,000 texts across writing styles, AI systems and attempts to evade detection. These are Grammarly’s statements about a third-party leaderboard. We have not reproduced the benchmark, and a benchmark result does not tell you how the tool will behave on your specific document.

Grammarly is also unusually direct about the limits. Its support guide says the score “should not be used as an objective source of truth”, that the detector “is not 100% accurate”, and that it “cannot provide a definitive conclusion”. The product page repeats that you should never rely on an AI detector alone to decide whether AI was used. If you are evaluating someone else’s writing, that is the part of Grammarly’s documentation to take most seriously.

One more point for anyone comparing scores across tools: Grammarly says it uses a proprietary in-house model, so a percentage from Grammarly may not match what Turnitin, GPTZero or Copyleaks report on the same text. Grammarly believes the scores are directionally similar, but it explicitly tells users not to assume a professor’s detector will show the same number.

Where it falls short, in Grammarly’s own words

These limitations come from Grammarly’s documentation rather than from our own testing.

• Short passages: Grammarly says shorter texts are harder to score accurately than longer ones. Very short samples give the model little to work with.
• No explanation of flags: the free tool reports a percentage and nothing else. You cannot see which sentences drove the score unless you use the paid agent.
• False positives are possible: Grammarly says its model is tuned to minimize false positives and expects them to be rare, but also acknowledges that human writing whose patterns resemble AI output can still be flagged. Formal, highly structured or heavily edited prose is the kind of writing most likely to sit in that grey area.
• Edited AI text: Grammarly states that its own rewriting agents, including its paraphraser and humanizer, produce text that will raise the AI score. Other generative rewrites behave the same way. The flip side is that any detector working from statistical patterns can miss AI text that has been substantially reworked by a person.
• Scores differ between tools: as above, a Grammarly result is not a prediction of what a different detector will say.
• Training data: Grammarly’s product page describes continuous model updates, while one of its FAQ answers on the same page describes training texts created before 2021. We are recording both statements as published; we cannot verify which reflects the current model.

Who should use it, and for what

The free detector is a reasonable self-check for your own writing. If you drafted something with AI assistance and want a rough sense of how it reads before submitting, a quick scan tells you whether the text leans heavily on patterns that detectors associate with AI. Grammarly positions the tool exactly that way: as a pre-submission gut check with guidance on responsible AI use, including a citation generator for disclosing AI assistance.

The paid agent is more useful if you need to act on the result, because it shows which passages are driving the score. Writers who use Grammarly’s own generative features should expect those passages to be flagged; Grammarly says so directly and suggests using agents for feedback you apply by hand if being flagged is a concern.

What the tool is not, by Grammarly’s own account, is proof. Educators and editors should not treat a percentage from any detector, including this one, as evidence of misconduct. Grammarly recommends using detection as one part of a broader approach, alongside conversation, drafts and tools such as its Authorship feature, which records how a document was written. If you want to understand which detectors institutions actually rely on, our guide to what AI detector professors use covers that landscape.

Can AI humanizers get past the Grammarly AI detector?

We do not know, and we will not pretend to. Our humanizer scorecard measures each tool against five detectors: GPTZero, Winston AI, ZeroGPT, Copyleaks and Originality.ai. Grammarly is not one of them, so we have no measured bypass rate for any humanizer against Grammarly, and we are not going to invent one. Grammarly itself says it does not offer a feature to lower a document’s AI score, and that rewrites from its own paraphrasing tools raise the score rather than reduce it.

What we can say is how humanizers performed on the detectors we did test. Walter Writes AI holds the top overall position in our rankings for readability and detector results across those five tools. Undetectable AI, StealthWriter and HIX Bypass are the other tools most readers compare, and BypassGPT, Humbot and Phrasly each have individual reviews with their scores. If Grammarly specifically matters for your use case, run your own text through the free detector before and after editing rather than relying on anyone’s generalization, ours included.

The bottom line on the Grammarly AI detector

Grammarly’s AI detector is easy to access, free in its basic form, and backed by a strong benchmark claim from Grammarly. It is also, by Grammarly’s own description, an estimate rather than a verdict: it does not explain its flags in the free version, it is weaker on short text, its scores are not comparable with other detectors, and the company says plainly that it should not be the sole basis for a decision. Use it to check your own drafts and to understand how your writing reads. Do not use it, or any single detector, to judge someone else’s work.