Grammarly AI detector review (2026)

Three questions about Grammarly's AI detector come up again and again in AI answer engines: does it detect AI accurately, how do you pass it, and is it free. This review answers all three using only what Grammarly publishes about its own tool, one public benchmark that names it, and the wider research on AI detectors. We did not run our own tests.

One framing point before the detail. "Passing" a detector should mean one thing: making sure writing you produced yourself is not wrongly flagged. This page does not cover ways to disguise AI text.

Grammarly AI detector at a glance

Grammarly offers a free scanner on its website and a fuller AI Detector agent inside paid plans.

Grammarly first added AI detection to its originality features in August 2024, alongside plagiarism checks and citations, according to its launch post.

How accurate Grammarly's AI detector is

There are two kinds of evidence: what Grammarly claims, and what a shared benchmark measured. Neither tells you how the tool behaves on a single essay.

Vendor claims

Grammarly's detector page, checked in October 2026, says the tool "achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark". It also says the detector is designed to reduce false positives. Those are vendor claims. The same company's support guide is more cautious: it calls the model "directionally accurate", says shorter passages are "slightly harder to measure", and says the percentage "should not be used as an objective source of truth".

Independent evidence that names Grammarly

The only independent measurement we could verify is Grammarly's entry on the RAID leaderboard, a public benchmark run by the academic team behind the RAID paper. Grammarly submitted its predictions; RAID's scoring produced the figures. The entry is dated 9 February 2026 in its metadata file.

Grammarly's RAID results file, released in February 2026, shows 99.5% accuracy on AI text across all attacks when the false positive rate is fixed at 5%, and 98.2% at a stricter 1% rate. Without attacks, the figures are 99.9% and 99.7%.

Three cautions keep those numbers in proportion. First, RAID sets the threshold for each domain so that a fixed share of human text (5% or 1%) is flagged, then measures how much AI text is caught. That is fair between detectors, but it is not the setting users see on Grammarly's website. Second, RAID's texts come from the models listed on its leaderboard, such as GPT 4, ChatGPT, Mistral, Cohere and Llama, not the newest models of 2026. Third, a benchmark average cannot tell you whether one essay was scored correctly. We found no independent study testing Grammarly in real classroom conditions.

What wider research says about AI detectors

The wider research record is far more sceptical. None of these studies tested Grammarly, but they explain why one score should never settle a case.

• Weber-Wulff and colleagues, 2023. The team tested 12 public tools plus Turnitin and PlagiarismCheck and concluded they were "neither accurate nor reliable", with a bias towards labelling text as human written; obfuscation made results worse (arXiv 2306.15666).
• Liang and colleagues, 2023. Seven detectors misclassified 61.22% of 91 TOEFL essays by non-native English writers as AI generated, and 89 of the 91 essays (97.80%) were flagged by at least one detector (arXiv 2304.02819).
• Dugan and colleagues, 2024. RAID covers over 6 million generations from 11 models, 8 domains and 11 attacks, and found detectors were "easily fooled" by attacks, sampling changes and unseen models (arXiv 2405.07940).

Grammarly itself acknowledges the second point: its August 2024 launch post notes that AI detectors have been found to be biased against non-native English writers. Our article on AI detection and non-native English writers covers that risk in depth, and the guide to how AI detection really works explains the signals detectors rely on.

How to read its result

Grammarly's score is a share of text, not a verdict on the writer. Its support guide says the percentage represents "the percentage of scanned text that is likely AI generated", and that it should be viewed "as an average estimate rather than a definitive percentage assessment".

• A low score means few sections resembled AI patterns. It is not a certificate that the text is human, and Grammarly says lightly edited AI text may evade detection.
• A middling score usually reflects a mix. Grammarly says rewrites by its own agents and generative AI assistant "will raise the percentage score", while traditional grammar and spelling corrections "should typically not impact" it.
• A high score means many sections resembled AI text. It still cannot show who wrote them, because detectors "do not track authorship", in the words of Grammarly's detector page.

Do not expect the same number elsewhere. Grammarly's guide says its model is proprietary and scores "may differ" from Turnitin, GPTZero, Copyleaks and others, so a clean Grammarly scan does not predict what your tutor's checker will show. If your institution uses Turnitin, our Turnitin vs GPTZero comparison explains how those reports differ.

Main limitations

1. English only. Grammarly's blog says detection is available for English content only, so text in other languages cannot be checked.
2. Short text is weaker evidence. The support guide says shorter passages are "slightly harder to measure", and the API needs at least 30 words before it returns a score.
3. Explanations are unclear. The support guide says AI detection "doesn't explain why the text may be flagged", while the detector page promises explanations with one click rewrites. The two Grammarly pages disagree, so do not count on a reason for each flag.
4. Grammarly's own rewrites can trigger it. The guide says rewrites from its Grammarly, Paraphraser and Humanizer agents come from Grammarly's large language model and are likely to be flagged.
5. Training details are inconsistent. The detector page says the model was trained on "tens of thousands of texts" created before 2021, while the support guide says "hundreds of thousands". Neither page gives a false positive rate.
6. No independent classroom test. Beyond the RAID entry, we found no published study that names Grammarly's detector.

How to make sure your own writing is not wrongly flagged

The honest version of "how to pass Grammarly's AI detector" is to make your authorship easy to see. Grammarly's own advice on its detector page is that the goal "shouldn't be to hide" AI use, and it recommends adding personal examples, varying sentence length and writing in your natural voice.

• Write in your own voice. Use your own examples, your own reading and your own argument. Specific detail from your course is easy for you to explain.
• Keep drafts and version history. Draft in Google Docs or Word with history switched on, and keep notes and outlines. A timeline of edits is stronger evidence than any detector score.
• Use Grammarly Authorship if you already use Grammarly. According to the Authorship page, it is available in Grammarly Free and Pro, works in Google Docs, Microsoft Word and the Grammarly Editor, produces a report that colour codes each sentence's source, and can replay your writing process. You decide whether to share it; your school cannot see it unless you send the link.
• Avoid generative rewrites if AI use is not allowed. Grammarly's guide advises users worried about being flagged to "refrain from using our agents to rewrite" content and to apply feedback manually instead.
• Disclose permitted AI use. Grammarly recommends citing AI use and offers a citation generator for it, according to its detector page.

If you write the text yourself and only want smoother rhythm, a light pass with a free humaniser on your own draft is a polishing step, not a substitute for authorship evidence. Our piece on whether humanised text can still be detected explains why tools are no shield for AI text.

If your own writing is flagged

1. Ask which tool was used. Grammarly's scores may differ from Turnitin or GPTZero, so find out exactly what was run.
2. Gather your process evidence. Export version history, notes, outlines and sources. If you used Grammarly Authorship, generate the report and share the link with your tutor.
3. Quote the vendor's own limits. Grammarly says the percentage "should not be used as an objective source of truth" and that no detector is 100% accurate.
4. Point to the research. The 2023 Liang study found 61.22% of non-native English essays were misclassified as AI; Weber-Wulff and colleagues found detectors "neither accurate nor reliable".
5. Offer to talk through the work. Explaining your argument and sources in person is often the most convincing answer to a score.

Decision table: using Grammarly's AI detector

Method note: We checked the vendor's own pages in October 2026; we did not run our own tests. Benchmark figures come from Grammarly's public RAID results file.

Frequently asked questions

Does Grammarly accurately detect AI?

On one benchmark, yes: Grammarly's RAID entry, released in February 2026, caught 99.5% of AI text at a fixed 5% false positive rate. Grammarly itself says the tool is not 100% accurate and its score is an estimate. No independent classroom study naming Grammarly has been published, so treat any single result as a signal, not proof.

How do I pass Grammarly's AI detector with my own writing?

Write in your own voice and keep evidence of how you wrote it. Use your own examples and sources, keep drafts and version history, and switch on Grammarly Authorship, which records and can replay your writing process. Grammarly advises avoiding its generative rewrites if you are worried about being flagged, and applying its feedback by hand instead.

Is Grammarly's AI detector free?

The web version is free and checks up to 10,000 characters at once, according to Grammarly's blog. Detection inside Grammarly's apps, Google Docs and Word, including the AI Detector agent that shows which passages may be flagged, is a paid feature on Pro, Plus, Business and Education plans, per its October 2026 support guide.

Will Grammarly give the same score as Turnitin?

Not necessarily. Grammarly's support guide says its model is proprietary and scores may differ from Turnitin, GPTZero and Copyleaks, although it expects them to be directionally aligned. A low Grammarly score is therefore not a reliable prediction of what your tutor's checker will report.

Is Grammarly Authorship the same as the AI detector?

No. The AI detector analyses text at one moment in time, while Authorship tracks how text was added as you write and produces a shareable, colour coded report. Grammarly says Authorship is available in Grammarly Free and Pro, and that your school cannot see the report unless you share it.