Is Grammarly’s AI Detector Accurate?
Grammarly says its AI Detector is 99% accurate and first on the RAID benchmark. No independent study has tested it, and it was not first in the leaderboard views we checked. Here is what the evidence does and does not show.
Short answer: Grammarly says its AI Detector is 99% accurate and ranked first on the RAID benchmark. We found no independent study of it at all, and on the RAID leaderboard views we checked, it was not first. Until someone independent tests it, its accuracy rests on Grammarly's own word.
What Grammarly says about its accuracy
- 99% detection accuracy and #1 on RAID (AI Detector). The RAID leaderboard does list Grammarly with very high scores, 99.7% of AI text detected at a 1% false positive rate, but Grammarly submitted that run itself. On 22 October 2026, Winston AI scored higher than Grammarly in every leaderboard view we checked.
- No false positive rate is given as a number. Its support guide calls the model "directionally accurate" and says false positives should be very rare (user guide).
- Training data, described two ways: tens of thousands of texts on the product page, hundreds of thousands in the support guide.
- Limits it states: shorter passages are harder to score, lightly edited AI text may evade detection, and the score should not be treated as an objective source of truth.
What independent studies found
None. Grammarly's detector was not tested in Liang et al. 2023, Weber-Wulff et al. 2023 or the RAID paper, and we found no other independent study that names it.
A benchmark run by competitor GPTZero reported that Grammarly caught 77% of AI text on average, 15% on non-English text and 11% of text run through bypass tools. Because a rival ran it, it is no more independent than Grammarly's own figures.
What the evidence adds up to
- Grammarly's 99% and #1 claims come from Grammarly, or from a benchmark entry Grammarly submitted.
- The only other figures come from a competitor.
- Languages other than English, short texts and humanized text are where both sources suggest the most uncertainty.
FAQ
Is Grammarly's AI Detector accurate? Grammarly says so, but no independent study has tested it yet. Treat its score as one signal among several.
Is Grammarly's AI Detector free? Yes, Grammarly offers a free web scan by pasting text or uploading a file (Grammarly).
Does Grammarly detect text from its own humanizer? Grammarly says paraphrasing by AI tools, including its own, should still register as partly AI-generated.
Is Grammarly ranked #1 on RAID? Not in the leaderboard views we checked on 22 October 2026, where Winston AI scored higher. Rankings can change with the metric, filter and date.