Grammarly's plagiarism checker is a genuinely different tool from an AI-writing detector, even though students often expect one to do the other's job. Here's what it actually compares your text against, and where it draws its own limits.
Trusted by students at
Grammarly states its plagiarism checker cross-references submitted text against over 16 billion web pages alongside ProQuest's academic and periodical databases — meaning it's checking for matching text against both the open web and a large academic source library, which is a genuinely broad comparison set for a text-matching tool.
That makes it reasonably strong at catching copy-pasted or lightly reworded text lifted from published sources. Like any similarity checker, though, a match doesn't automatically mean misconduct — properly quoted and cited material will also show up as a 'match', which is expected and normal.
Grammarly is upfront that detecting AI-generated writing is a separate, harder problem than detecting copied text, and the company itself acknowledges that increasingly human-like AI writing makes reliable detection difficult. If you're checking whether text might read as AI-generated rather than copied, that's a different check, not the plagiarism checker's job.
Grammarly also states your submitted text isn't stored in a way that's publicly searchable by others — a privacy point worth knowing, though it's separate from the accuracy question.
Grammarly offers a basic plagiarism check at no cost, with fuller functionality bundled into Grammarly Premium. If you only need an occasional check rather than an ongoing writing assistant subscription, it's worth weighing against a dedicated free plagiarism checker for that specific task.
See matched passages and sources at a glance — free, no account needed for your first check.
Check My Essay Free →