Does Grammarly Detect Plagiarism & AI Content?

Does Grammarly detect plagiarism?

Yes. Grammarly’s plagiarism checker compares your writing against billions of web pages and a database of academic papers, then reports a similarity percentage and shows which passages match existing sources. It is part of Grammarly Premium — the free plan flags that potential plagiarism exists but does not show you the details.

It is a similarity checker, not a verdict on intent: a high score can simply mean you quoted or paraphrased a source without citing it. The fix is almost always proper attribution, not rewording to hide the match.

Does Grammarly detect AI writing?

Yes — separately. Grammarly added an AI detection feature that estimates the percentage of a document that appears AI-generated, sitting alongside the plagiarism and grammar tools. Like the plagiarism checker, full AI detection is a Premium feature.

It scores the same statistical fingerprints other detectors use — predictable word choice (low perplexity) and uniform sentence length (low burstiness) — and returns a single percentage rather than a sentence-by-sentence map.

How accurate is Grammarly’s detection?

Both tools are probabilistic. The plagiarism checker is only as good as the sources it can see, and the AI detector — like every AI detector — produces false positives (human writing flagged as AI) and false negatives (edited AI passing as human). Clean, formulaic, or non-native English writing is the most likely to be flagged in error.

Grammarly’s AI detector is also not the same tool your school uses. Many institutions run Turnitin, which has its own model and database. A clean score in Grammarly does not guarantee a clean score in Turnitin, and vice versa.

Check your text free before you submit

You don’t need a paid plan to get an early read. Humanit’s AI detector is free every day: paste your text and it returns a 0–100 AI-likelihood score, a verdict, and subscores showing exactly which signals — phrasing, rhythm, stock vocabulary — are pulling the score up. That tells you where to look before anyone else checks it.

What to do if your writing is flagged

For a plagiarism flag, add the citation — that’s what it’s actually telling you to do. For an AI flag on your own writing, vary your sentence length, cut stock transitions like “moreover” and “in conclusion”, and add concrete, specific detail. For AI-assisted drafts, run the text through Humanit’s humanizer to restructure the phrasing, then re-check with the detector. Always follow your school or employer’s rules on AI use — that responsibility is yours.