No single answer holds across every case. Grammarly's AI detector and Turnitin's AI detector score the same underlying signals, perplexity and burstiness in the writing, but each is trained on different data with a different flagging threshold. They'll agree on obviously AI-generated text and disagree on the borderline paragraphs that actually matter.
What Grammarly's AI detector checks
Grammarly's detector is built into a broader writing-assistant product, scanning submitted text for the statistical fingerprints associated with large language model output and returning a percentage estimate of how much reads as AI-generated.
What Turnitin's AI detector checks
Turnitin's detection layer works on the same class of signal but is trained specifically against academic writing and integrated into the institutional similarity report students and instructors actually see. The full mechanics are in what AI detector does Turnitin use.
Why two detectors trained on the same problem still disagree
Detection models are trained on different corpora, tuned against different generations of AI models, and calibrated with different sensitivity thresholds. A paragraph that trips Turnitin's threshold might sit just under Grammarly's, and neither result is more objectively correct than the other. Agreement between two independently-trained tools is a meaningfully stronger signal than either score alone.
Which one actually decides your grade
If your institution licenses Turnitin, that's the report your instructor reads, regardless of what Grammarly says about the same paragraph. Grammarly's score is useful as a second opinion or a pre-submission check, but it carries no institutional weight on its own.
For the accuracy baseline on AI detectors as a category, not just these two, see are AI detectors accurate.
Cross-check your own draft against a third, independent read using a free AI detector before you rely on either single score.
Frequently asked questions
Should I trust Grammarly's score over Turnitin's?
Only as a second opinion, never as the final word if your institution licenses Turnitin. Turnitin's report is the one that carries academic weight.
Why did Grammarly flag my writing but Turnitin didn't (or the reverse)?
Different training data and thresholds. Borderline text sits differently relative to each tool's cutoff, and disagreement between the two is common, not a sign either one is broken.
Does Grammarly's AI detector check for plagiarism too?
Grammarly's plagiarism checker is a separate feature from its AI-detection score, the same split that exists on most tools in this category. Check both independently if that matters for your submission.
