Turnitin vs Grammarly AI Detection: Scores and Limits

Turnitin and Grammarly can both flag writing as potentially AI-generated, but a Grammarly result does not predict the report your institution will receive. Choose by the workflow you need; there is no current controlled head-to-head test in this article that establishes an accuracy winner.

Evidence and disclosure — reviewed September 8, 2026. This is a source-based editorial review, not our own accuracy benchmark. Humanizer AI sells writing and detection tools, so we have a commercial interest in this topic.

What each score tells you

Turnitin separates its AI writing indicator from its similarity score. Its AI percentage concerns qualifying prose, and its guide warns against using the result alone to penalize a student. Turnitin's report guide explains the distinction.

Grammarly's public detector estimates how much text resembles AI writing. Its own FAQ acknowledges that a detector cannot conclusively establish authorship. Grammarly's AI detector describes that output.

Those are different reports produced by different systems. A percentage of qualifying prose is also a different quantity from the probability that a writer used AI. Do not average two percentages to create an alleged consensus score.

Compare access and purpose

Grammarly distinguishes its paid in-product features from the public checker. Check the AI Detector user guide before paying for a particular workflow.

Why this comparison has no accuracy league table

An honest comparison needs the same human and generated documents, current product versions, identical input handling, a defined decision threshold, and separate error counts. Marketing percentages from different corpora cannot supply those controls. This article does not establish that either service detects edited text better or produces fewer false accusations.

For example, imagine an original essay with a long reference list and a short body. Comparing a whole-document output with an analysis of qualifying prose can produce disagreement even before model differences matter. That is an interpretation example, not a test result.

What to do when the results disagree

Keep the original draft, source notes, and version history. Identify the passages each service highlights and check that both received the same text. Discuss the evidence with the person applying the course or publication policy. Repeatedly rewriting until a score drops can destroy useful evidence of your writing process.

Verdict: use your institution's report to understand its review process and a personal checker as optional draft feedback. Neither score grants permission to use AI or proves misconduct. For a closer look at the consumer workflow, read our Grammarly review. Our Turnitin checker explanation and detection methodology explain what Humanizer AI can and cannot report.