Direct answer
Grammarly can flag text that its AI detector considers likely AI-generated, but it cannot confirm authorship, and its percentage is a probabilistic signal rather than a verdict. Grammarly's detector returns an overall score plus sentence-level highlights, and those results routinely shift between tools and between drafts of the same text [1]. If you need to know what your institution will actually see, the meaningful check is the report your university runs — a Turnitin AI writing report paired with a similarity report — not a writing assistant's estimate [1].
Can Grammarly Detect AI-Generated Text Accurately?
Grammarly does offer AI-detection functionality, and it is genuinely useful as a first-pass signal: it scans submitted text and returns an overall percentage with sentence-level highlighting so you can see which passages triggered the model [1]. The important caveat is what that number represents. It is the output of a statistical classifier estimating the likelihood that text resembles AI-generated prose, not a record of how the text was produced [2].
Accuracy is where the tool's limits become practical rather than theoretical. Independent evaluations of AI detectors consistently find false positives on genuinely human writing, and error rates climb when text is short, highly formulaic, or heavily edited [2]. Academic writing is unusually exposed to this problem because it is deliberately impersonal, hedged, and structurally predictable — exactly the stylistic features classifiers associate with machine output [2].
There is a second reliability issue that matters more for students: cross-tool disagreement. The same paragraph can be flagged by one detector and cleared by another, because each vendor trains on different data and applies different thresholds [1]. Treating any single detector's output as the authoritative answer to "did I write this?" misreads what the technology can do [2].
The practical takeaway is to use Grammarly as a diagnostic, not a decision. If it highlights three sentences, read those sentences and ask whether they sound like your voice or like a model's default register — that editorial judgement is more valuable than the headline percentage, and it is the part no detector can do for you [1][2].
What Does Grammarly's AI Detection Score Actually Tell You?
The headline percentage is best read as a proportion, not a probability of misconduct: it reflects how much of the submitted text the detector's model associates with AI-generated patterns [3]. That distinction matters because a score of, say, 40% does not mean "40% chance you cheated" — it means a substantial share of the text tripped the classifier's features, and the sentence-level highlights show you exactly where [3].
Length and context distort the number more than most users expect. Short samples give the model too little evidence, so results on a single paragraph are far less stable than results on a full essay, and quoted material, reference lists, and templated methodology sections can inflate a score without any AI involvement [3]. If you test a 200-word excerpt and then the full 2,500-word draft, expect the two figures to disagree — and trust the fuller document [3].
The score also has no institutional standing on its own. Your marker is not reading Grammarly's output; they are reading whatever report your university's submission system generates, and those systems present their own scores, thresholds, and sentence-level flags [3]. A clean Grammarly result therefore does not guarantee a clean institutional result, and a bad one does not prove a problem exists [3].
Read the score as a prompt for revision rather than a grade. The actionable information sits in the highlighted sentences: rewrite the flagged passages in your own phrasing, add specific evidence and citation, and re-check [3]. That workflow improves the writing and reduces detector sensitivity at the same time, which is the outcome most students actually want.
How Do You Verify an AI Score Before You Submit?
Verification means reproducing the check your institution will run, because that is the only score with consequences. Institutional systems report an overall AI percentage alongside sentence-level indicators, and they display a masked value rather than an exact figure when the signal falls below the vendor's confidence threshold — a detail students frequently misread as an error or a zero [4]. Knowing how that display behaves prevents a lot of unnecessary panic the night before a deadline.
Those platforms also separate two different questions that students tend to merge. The similarity report measures overlap with existing sources, including properly cited quotations, while the AI writing report measures stylistic probability — a document can show high similarity and zero AI, or the reverse [4]. Checking only one of the two leaves half the picture unexamined, and the two reports are read side by side by most markers [4].
Timing and file handling are the practical constraints. Institutional checks run against a student paper repository, so a draft submitted early can be matched later, and the report format depends on the file type you upload [4]. A verification step that mirrors those conditions — same document, same length, both reports returned — gives you information you can actually act on before the deadline rather than after it [4].
Finally, treat any single number as a starting point for a conversation, not a verdict. If a report flags text you wrote yourself, the useful response is to keep your drafts, notes, and version history, and to be ready to discuss your process with your tutor [4]. Detectors are probabilistic instruments; documented authorship is what resolves a dispute [4].
If you want the same view your marker will have — overall AI percentage, sentence-level flags, and the similarity summary in one place — turnitin0 runs a pre-submission check that returns a Turnitin AI detection report and a similarity report together, typically in under 15 minutes, 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 the same as Turnitin's?
No. They are separate products built on different models and thresholds, which is why the same draft can score differently in each [1][4]. Turnitin's AI writing report is the one most institutions actually read.
Does a high Grammarly AI score mean I will be penalised?
Not on its own. A high score indicates stylistic similarity to AI-generated prose, and it is a probabilistic signal rather than evidence of misconduct [2]. Your institution's own report and your documented writing process carry far more weight [4].
Why does my score change when I check the same text twice?
Detector output is sensitive to text length, editing, and threshold settings, so short or heavily revised excerpts produce unstable results [3]. Check the full document rather than a paragraph for the most consistent reading.
Can I lower an AI score without rewriting everything?
Yes, in most cases. Rewriting flagged sentences in your own phrasing and adding specific evidence, citation, and concrete detail reduces detector sensitivity while improving the writing itself [3].
What should I do if a detector flags text I wrote myself?
Keep your drafts, notes, and version history, and be prepared to discuss your process with your tutor [4]. Documented authorship is what resolves a detection dispute; no detector output settles it.