What Grammarly’s detector is
Grammarly’s AI detection reports the percentage of a document that reads AI generated, positioned carefully as guidance rather than proof: the company’s own copy stresses that no detector is fully reliable. It sits inside the broader Grammarly product rather than standing alone, which shapes both its audience (writers checking themselves) and its incentives (reassurance over accusation).
How it reads in practice
On our samples, Grammarly’s detector behaves conservatively. Clearly machine text reads as AI, but borderline and edited samples read human more often than aggressive tools like ZeroGPT would score them. For a tool aimed at writers self-checking, conservative is a defensible design: fewer false alarms on honest work, at the cost of waving through some lightly edited machine text. It also means a clean Grammarly read is weak evidence in any enforcement context, and a teacher relying on it to clear or accuse a student is using a consumer reassurance feature for a forensic job.
Grammarly’s rewrite features are themselves machine generation. Use them heavily and every detector, including Grammarly’s own, will correctly read the result as more machine-like. Light correction of typos barely moves any dial; full-sentence rewrites move all of them. The detector is not malfunctioning. It is describing what happened to the text.
Compared with the field
Detector disagreement is normal, and Grammarly disagrees in a consistent direction: it under-flags relative to ZeroGPT and reads close to our own dial on clear cases while resolving borderline cases toward human. If a text you care about splits the tools, that split is the finding: the text is genuinely borderline and no single score should decide anything. Cross-checking costs nothing; this site needs no account, and the comparison guide in best AI detectors 2026 maps the rest of the field.
The verdict
Accurate enough for its actual job, which is helping a writer understand how their text reads. Not built for, and not honest to use for, screening or enforcement. If you need a reading with explicit uncertainty bands and copy that tells you what the score cannot prove, that is the design brief of the detector on this site. If you need sentence-level highlighting, Sapling does that better than either of us.
The protocol behind our observations
So you can weigh our claims about Grammarly's behavior, here is exactly what we ran. Three sample sets: human essays and articles written before 2022, unedited longform from the current GPT, Claude and Gemini models, and AI drafts given ten to twenty minutes of human revision each. Every sample went through Grammarly's public detector and this site's engine in the same session, verdicts recorded against ground truth. Grammarly cleared the human folder with the fewest false flags in our runs, caught most unedited machine text, and resolved the revised drafts toward human more often than the rest of the field: the conservative profile described above. Sample sizes were modest and curated by us, which is the same caveat we attach to every vendor's internal testing, so treat this as a documented observation rather than a benchmark. Better yet, replicate it: the protocol takes an afternoon and your own writing is the ground truth that matters.