Does Grammarly Detect AI Writing? What It Actually Checks

The short answer: yes, but not conclusively, and Grammarly says so itself.

Grammarly offers a free AI detector that gives you a score for how much of a text appears AI-generated. Its paid Pro plan adds an "AI Detector agent, which goes beyond a score." And Grammarly Authorship, available on the free and paid plans, records how a document was written: what was typed, what was pasted, and what was AI-assisted. What none of these can do is prove whether AI was used. In Grammarly's own words, "currently there is no AI detector that can conclusively or definitively determine whether AI was used to produce text."

If you're a student worried about getting falsely flagged, a writer trying to verify your own work, or a teacher trying to catch AI submissions, the details matter more than the headline. Here's what Grammarly documents, what independent research says about detectors like it, and how to test it on your own writing.

TL;DR: Grammarly Detects AI, but a Score Isn't Proof

Quick summary:

• A free AI detector on grammarly.com: paste text, click scan, and get a score for how much of it appears to be written with AI.
• An AI Detector agent for Grammarly Pro subscribers, which Grammarly describes as going beyond a score.
• Authorship, on Grammarly Free and Pro, which records your writing process in Google Docs, Microsoft Word and the Grammarly Editor, and can be enabled on Canvas LMS submissions.
• Strong accuracy claims: Grammarly says its detector "achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark."
• An explicit limit: Grammarly also says "no AI detector is 100% accurate" and that AI detection "should never be used as a standalone verification method."

Bottom line: treat a Grammarly score as one signal, never as a verdict, and remember it is not the detector your school runs. If you're checking a draft, check it against the tool that will actually grade it. If you're revising AI-assisted text, SupWriter can help it read naturally, with a detector pre-check built in.

How to Test Grammarly's AI Detector Yourself

We haven't published a controlled test of Grammarly's detector, and you shouldn't take anyone's unaudited numbers on faith, including ours. What you can do is run a small, honest test in an afternoon. It won't be a benchmark, but it will tell you how the tool behaves on your kind of writing, which a benchmark can't.

The Dataset

Ten human-written samples you're sure about. Your own older essays and emails are ideal, especially anything written before ChatGPT's release in late 2022. Deliberately include a mix of quality, from polished pieces to rough first drafts with typos.

Ten AI-generated samples on similar topics. Generate them with whatever models you actually use, from a single prompt, with no manual editing, so you are testing raw AI output.

Five lightly edited AI samples. Take five of the AI pieces and spend five minutes on each: swap some words, split a long paragraph, add a detail. This is where detectors behave most differently.

Content Types

Cover the kinds of writing you actually produce:

• Academic essays (argumentative, analytical, and research-style)
• Blog posts (tech, marketing, lifestyle, how-to guides)
• Professional emails (cold outreach, internal communication, follow-ups)
• Creative writing (short fiction, personal narratives, opinion pieces)

Formal, formulaic writing is where false positives tend to cluster, so if you write academic or business prose, over-sample it.

What to Control For

Paste plain text rather than formatted documents. Keep samples in a similar length range, a few hundred words or more, because very short texts give any detector little to work with. Run each sample twice to see whether the score moves. Record the score, the highlighted passages, and the date, and take screenshots, because detectors update their models without notice.

Then do the same with the detector that actually matters to you, whether that is your school's Turnitin setup, GPTZero, or a client's tool. The comparison is the point.

What Grammarly Says About Its Detector's Performance

Let's start with Grammarly's own claims, then add the independent evidence.

Overall Accuracy

Grammarly says its detector "achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark, outperforming other leading AI detectors." It says the model "was trained on tens of thousands of texts, including both human-written and AI-generated text," and that it analyzes each section of a text "for language patterns that are often linked to AI-generated writing" (Grammarly AI detector).

RAID is a real and demanding benchmark. Published at ACL 2024, it contains over 6 million generations spanning 11 models, 8 domains, 11 adversarial attacks and 4 decoding strategies. Its authors also found that the detectors they evaluated were "easily fooled by adversarial attacks, variations in sampling strategies, repetition penalties, and unseen generative models" (Dugan et al., 2024). A strong benchmark ranking is a good sign. It is not a guarantee about your paragraph, which is why Grammarly pairs the claim with its own warning that no detector is 100% accurate.

False Positives: Who Gets Flagged

Grammarly's detector page says the tool is "designed to avoid wrongly flagging human-written text." It doesn't give a false-positive rate on that page.

Independent research shows why that number matters. In a 2023 Stanford study, seven detectors flagged, on average, 61% of 91 human-written TOEFL essays by non-native English speakers as AI-generated, while essays by native speakers were classified accurately (Liang et al., 2023). That study couldn't have tested Grammarly's current detector, but it shows how badly a detector can misfire on one group of writers while looking fine on another. Clear, formal, carefully constrained prose shares surface traits with AI output, and that is exactly the writing that careful students and second-language writers produce.

Which AI Models It Catches

Grammarly's detector page doesn't publish results by model. The general pattern from independent research applies to every detector: accuracy drops on models, settings and domains a detector wasn't trained on. That is one of RAID's central findings, and it is why a detector's performance on last year's ChatGPT tells you little about this year's Claude or Gemini.

How Grammarly Compares to Dedicated Detectors

Every major detector now makes a headline accuracy claim. Here's what each vendor says and what access looks like, as published on their own pages in September 2026:

Sources: Grammarly, GPTZero, Originality.ai, Turnitin AI detection FAQ.

None of these claims is a head-to-head audit on your kind of writing. They were measured by each vendor on its own terms, which is exactly why the do-it-yourself test above is worth an afternoon.

Where Grammarly's AI Detector Falls Short

Grammarly is a genuinely good writing assistant, and its detector claims are strong. But some limits come with the territory, and no benchmark score removes them.

It's a Grammar Checker First, Detector Second

Grammarly's core product is writing help, and its own pages steer you toward process evidence rather than detection when authorship really matters. The Authorship page puts it bluntly: "AI detectors only guess from the final draft. Authorship records your actual writing process—typed, pasted, AI-assisted." It can replay your process and produce a shareable report that color-codes each sentence by source (Grammarly Authorship). If Grammarly itself treats the detector score as the weaker signal, so should you.

Training Data Can Lag New Models

AI models evolve fast, and every detector is trained on the outputs that existed when it was built. Grammarly doesn't list which models its detector was trained on. When a new model or an unusual generation setting arrives, any detector can miss it until it is retrained, which is the "unseen generative models" problem the RAID authors describe.

Edited AI Text Is Harder for Every Detector

Light editing changes the statistical patterns detectors rely on. A study of 14 detection tools, including Turnitin, concluded that "the available detection tools are neither accurate nor reliable" and that "content obfuscation techniques significantly worsen the performance of tools" (Weber-Wulff et al., 2023). There's no reason to expect Grammarly's detector to be immune, and Grammarly doesn't claim it is.

A Score Is Not a Verdict

A detector score is a probability estimate, not a finding of fact. Grammarly says AI detection "should never be used as a standalone verification method," and Turnitin says its own AI percentage "should not be used as the sole basis for action." Even a detector with a low false-positive rate will wrongly flag some human writers when it scans thousands of documents; our explainer on how AI detection works walks through the base-rate math.

What Actually Works for Detecting (and Humanizing) AI Text

Depending on which side of the detection question you're on, here's what we'd recommend.

If You Need to Detect AI Writing

Don't rely on any single detector, Grammarly's included. A more defensible approach looks like this:

1. Use the detector that actually governs the decision. In many schools that's Turnitin, which needs at least 300 words of prose, shows 1–19% scores only as an asterisk, and tells instructors not to use its percentage as the sole basis for action.
2. Look at process, not just the final text. Version history, Grammarly Authorship reports, drafts and notes say far more about who wrote something than any score.
3. Get a second opinion if you must. GPTZero and Originality.ai both offer free scans. If two tools disagree, that disagreement is information, not a tie to break.

We cover free alternatives in more depth in our AI detector tools comparison.

One important caveat: no AI detector is 100% accurate. Even the best tools make mistakes. We strongly recommend against using any single detector result as the sole basis for an accusation of AI use, especially in academic or professional contexts.

If You Need AI Text to Sound Human

Maybe you're on the other side of this. You use AI to draft content, and you need it to read naturally. That's where SupWriter comes in.

We built SupWriter to humanize AI-assisted drafts. Not by spinning words or adding random errors, but by restructuring sentences, varying rhythm, and keeping your meaning intact. Before you submit, check the result with SupWriter's built-in AI detector. It is a pre-check based on one scoring model, not Grammarly's detector or the one your school uses, and results vary by detector, text length and topic, so no tool can guarantee an outcome (how we test). And if your school or employer restricts AI use, check the policy before you rely on any of this.

If you're currently using Grammarly and wondering about alternatives that handle both grammar correction and AI humanization, check out our Grammarly alternative comparison.

Frequently Asked Questions

Does Grammarly have a built-in AI detector?

Yes. Grammarly offers a free AI detector on grammarly.com that shows how much of your text appears to be AI-generated, and its Pro plan adds an AI Detector agent that goes beyond a single score. Separately, Grammarly Authorship records how a document was written. Grammarly itself says no detector can conclusively determine whether AI was used, so treat the result as a rough indicator rather than a definitive answer.

Can Grammarly detect ChatGPT writing specifically?

Grammarly's detector estimates how much of a text resembles AI-generated writing in general; its results page doesn't identify which model wrote something. It can flag ChatGPT output, but like every detector it can also miss AI text, especially text that has been edited. Independent research shows that editing and paraphrasing significantly worsen detector performance across the board.

Is Grammarly's AI detection free?

The web-based AI detector is free to use. Grammarly's plans page lists "Detect AI generated text" as a paid-plan feature inside the product, and its detector page says Grammarly Pro adds the AI Detector agent. Authorship is available on both the free and Pro plans. Prices vary by region, so check Grammarly's plans page for what you'd pay.

Why does Grammarly flag my writing as AI when I wrote it myself?

This is the false positive problem, and it affects every detector. Formal, structured writing and writing by non-native English speakers are the most exposed, because clear, predictable prose shares surface traits with AI output: clean grammar, logical flow, consistent tone. Ironically, careful writing gets flagged precisely because it is careful. If it happens to you, keep your drafts and version history, and consider using Authorship, which records your actual writing process rather than guessing from the final text.

What's the most accurate AI writing detector in 2026?

There's no neutral, current, head-to-head audit that settles it. Grammarly, GPTZero, Copyleaks and Originality.ai all claim top-tier accuracy, and Turnitin reports a false-positive rate under 1% for documents with over 20% AI writing. Benchmarks like RAID help, but performance on your writing can differ. For a practical breakdown of free options, see our AI detector tools comparison. Keep in mind that all detectors have limitations and none should be treated as infallible.

The Bottom Line

Grammarly is an excellent writing assistant, and yes, it detects AI writing: a free detector, a deeper Pro agent, and Authorship for process evidence. What it can't do, by its own account, is give you certainty.

That matters because false positives have real consequences for real people. Students accused of cheating. Writers questioned about their authenticity. Professionals losing credibility over a single score. Whatever detector you use, Grammarly's or anyone else's, treat the result as a prompt to look closer, not as proof.

If you need to detect AI text, use the detector that governs the decision and look at the writing process. If you need AI-assisted text to read like a human wrote it, give SupWriter a shot, and check the result before you submit. Either way, don't let one number do the job of judgment.

Related Resources

• Best Grammarly Alternative — For AI humanization
• Grammarly vs QuillBot 2026 — Head-to-head comparison
• Grammarly vs ChatGPT — Which creates more detectable text?
• Grammarly for Students — Best student alternative
• Best AI Humanizer Tools — Complete ranked list
• QuillBot Review 2026 — Features, limits and pricing