Direct answer
Grammarly's AI detector gives you a percentage estimate of how much of a text looks machine-generated, but it is not proof. Grammarly itself describes the result as an estimate rather than a definitive verdict, and the same is true of every major detector, including Turnitin's [1]. If the question matters — a submission, an appeal, a grade — you need a second, report-level check that shows which passages were flagged and why, not just a single number.
Can Grammarly Tell You If Something Was Written by AI?
Grammarly's AI detector analyses a pasted block of English text and returns an estimated share of it that appears AI-generated. It is a useful first pass: it is fast, free to try, and it will catch obvious cases of raw model output pasted straight into a document [1]. What it will not do is tell you who wrote a specific sentence, and Grammarly is explicit that the output is an estimate rather than a determination of authorship [1].
The reason is structural. No detector reads a text's origin; it reads statistical patterns — how predictable each next word is, how uniform sentence rhythm is, how evenly vocabulary is distributed. Turnitin describes its own indicator the same way: it reports the percentage of qualifying prose that its model flags, not the probability that a student cheated [2]. That number is an artefact of pattern matching, so it moves when the writing style moves, even if the author never changed.
There is also a coverage limit worth knowing before you rely on a single score. Turnitin only scores prose segments long enough to give its model something to work with — short answers, bullet lists, equations, and quoted material sit outside the calculation [2]. A Grammarly percentage computed over a whole document and a Turnitin percentage computed over qualifying segments are therefore measuring overlapping but not identical things [1][2]. Comparing the two numbers directly, or treating either as a verdict, misreads what both tools actually report.
Why Does Grammarly Say Text Is AI-Written When a Human Wrote It?
False positives are the most common reason people search this question, and they have identifiable causes. Detectors reward unpredictability, so writing that is unusually smooth, evenly paced, and grammatically regular can score as machine-like even when a person typed every word [3]. Academic conventions push writers in exactly that direction: formal register, parallel structure, signposting phrases, and a fixed citation rhythm all reduce the variation detectors look for.
Grammarly's own feature set can amplify the effect. Rewrite, paraphrase, and tone-adjustment tools are designed to regularise prose — that is their job — and regularised prose is precisely what a detector reads as machine-generated [3]. A student who drafted everything by hand and then ran the whole document through a rewriting feature has genuinely changed the statistical signature of the text, without any AI having written the argument. Non-native English speakers are disproportionately affected for the same reason: careful, rule-following phrasing looks uniform [3].
The practical consequence is that a flag is a prompt for a conversation, not a finding of misconduct. Turnitin's guidance to institutions is to read the highlighted passages, weigh the writer's drafting history, and consider the context of the assignment before drawing conclusions [2][3]. If you have your version history, your notes, and your earlier drafts, you have the evidence that matters — and it is evidence a percentage alone cannot override.
How Do You Verify an AI Score Before You Submit?
Verify with a report, not a number. A proper AI writing report shows an overall percentage alongside highlighted segments, so you can see exactly which sentences triggered the model and judge whether the flag is fair [4]. That segmentation is what lets you fix a real problem — a paragraph you did paste from a chatbot — without rewriting prose that was always yours.
Read the score the way the platform displays it. Turnitin shows *% instead of an exact figure when the AI score falls below its 20% confidence threshold, which signals a low-confidence result rather than a clean zero [4]. Many students misread that asterisk as a hidden high score. It is the opposite: it is the detector telling you it does not have enough signal to commit.
The most useful pre-submission step is a non-repository check that returns the same report format your institution sees, without adding your draft to Turnitin's student paper database. That gives you the segmentation, the threshold behaviour, and the similarity matches in one pass, so you can revise deliberately instead of guessing at what a detector disliked [4]. Do that check after your final edit, not before — rewriting after a check resets the signals you just measured [2][4].
A Grammarly percentage tells you that something in the text looked predictable; it cannot tell you which paragraph, why, or whether an instructor would agree. If you want the answer in the form that actually decides the outcome — a full report with flagged passages, score bands, and similarity matches — turnitin0 runs that check on your draft and returns the same two PDFs your university's system produces, typically in under fifteen minutes and without adding your file to the student paper database.
※ Turnitin0.com - Actual Turnitin AI Report Cover, Score, Flag And Similarity Summary
FAQ
Is Grammarly's AI detector accurate enough to rely on?
It is accurate enough to be a first signal and not accurate enough to be a verdict. Grammarly presents its output as an estimate, and detectors generally struggle with heavily edited human writing and with non-native English prose [1][3]. Treat a positive result as a reason to look closer, not as a conclusion.
Does using Grammarly's rewriting tools make my text count as AI-written?
Grammarly's rewriting features change the statistical signature of your prose, which can raise an AI score even when you wrote the original argument yourself [3]. The writing is still yours; the pattern the detector reads has shifted. Keep your draft history so you can demonstrate the difference.
Why did Turnitin show an asterisk instead of a percentage?
Turnitin displays *% when the AI score falls below its 20% confidence threshold — a low-confidence result, not a concealed high score [4]. It means the model did not find enough qualifying signal to report a firm figure.
Can I check my AI score before submitting without it going into Turnitin's database?
Yes. A non-repository pre-check returns the same report format without adding your document to the student paper database, so you can see flagged passages and similarity matches before your final submission [4]. Run it after your last edit so the report reflects the text you will actually hand in.
What should I do if my own writing is flagged as AI?
Read the highlighted segments first, then assemble your evidence: version history, notes, earlier drafts, and any source material [2][3]. Detector output is one input to an academic judgement, and documented drafting process is the strongest counter-evidence available to you.