Does Grammarly Detect Plagiarism & AI Content?

Does Grammarly detect plagiarism?

Grammarly’s plagiarism checker and its AI detector are separate products. The AI side — including how it differs from Grammarly Authorship — is on the review page. The plagiarism side is below.

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. The basic AI scan is free on Grammarly's website without an account; phrase-level detail and one-click rewrites of flagged text sit on paid plans. The Grammarly AI detector review covers the Authorship distinction in full, and has a free checker you can run without an account.

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.

Grammarly now owns GPTZero

On June 23, 2026, Superhuman, the company Grammarly became after buying the Superhuman email app, announced it was acquiring GPTZero. As of September 2026 the two detectors still run as separate products and still score differently: Grammarly reports the share of your text that appears AI-generated, while the GPTZero AI detector gives a probability that the document as a whole is AI-written. So a Grammarly result and a GPTZero result now come from the same company, but they are still two different measurements and will not always agree.

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.

Can Grammarly detect ChatGPT, Gemini, and Claude?

Yes — Grammarly’s AI detector is built to flag machine-generated text in general, so it will flag output from ChatGPT, Gemini, Claude, and other large language models. It scores the statistical pattern of the writing — how predictable the word choice is and how uniform the sentence length is — not which specific app produced it. Long, unedited AI passages are the most likely to be flagged.

The catch is that detection weakens fast once a human gets involved. AI text you have edited in your own voice, prompted to write loosely, or run through a humanizer is much harder to flag — and genuinely human writing sometimes gets flagged anyway. The percentage is a probability, never a confession.

Grammarly vs Turnitin: which score actually counts

This is the mistake that catches students out. A clean score in Grammarly does not mean a clean score in Turnitin: they are different models, trained on different data, run by different companies. Most schools and universities check submissions with Turnitin or a similar institutional tool — not with Grammarly.

So treat Grammarly’s plagiarism and AI scores as a private early-warning system that helps you find problems before you submit, not as the number that goes on your record. The only score that counts is the one your institution’s checker produces.

Grammarly free vs Premium for plagiarism and AI

For plagiarism, the free plan tells you potential plagiarism was detected but hides the details — the matched sources and the similarity percentage. Seeing those requires Grammarly Premium. The AI detector is more generous: its basic scan is free on Grammarly's website without an account and shows an overall percentage, while phrase-by-phrase detail and one-click rewrites of flagged text are on paid plans.

If all you want is a quick read on whether your text looks AI-generated, you don’t have to pay for it. A free AI detector returns a score and the signals behind it, which is usually all you need for a final edit before submitting.

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 with no account for up to 300 words: 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.

Does Grammarly flag paraphrased or AI-humanized text?

Lightly paraphrased AI text often still trips the detector, because basic synonym-swapping leaves the underlying statistical pattern — predictable word choice and even sentence length — mostly intact. Detectors score that pattern, not the individual words, so changing vocabulary alone rarely moves the score much.

What actually lowers an AI score is structural change: varying sentence length, breaking up uniform rhythm, cutting stock transitions, and adding specific detail. That is the real difference between a synonym-swapping paraphraser and a humanizer that restructures at the sentence level. Either way, re-check the result in a detector rather than assuming the edit worked — and remember detection is probabilistic, so no edit guarantees a clean pass.

What AI percentage is acceptable on an assignment?

There is no universal number. Schools, instructors, and platforms each set their own thresholds, and many treat any AI score as a prompt to look closer rather than an automatic penalty. A low percentage is not a guarantee of safety, and a single high number is not proof of misconduct — the score is a probability, not a verdict.

Because the threshold is set by your institution, the safest move is to ask what their policy actually is and to keep your drafts and version history as evidence of how you wrote the piece. Treat any detector percentage — Grammarly’s or anyone else’s — as a private signal to review your work, not as the official result.

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.