Turnitin vs Grammarly AI Detection: Which Is More Accurate?

If you're wondering whether to trust Turnitin or Grammarly for detecting AI-generated content, you're asking the right question. Both tools now offer AI detection, but they serve different purposes and deliver vastly different results.

The short answer: Turnitin is significantly more accurate for AI detection, especially in academic settings. Independent tests consistently show Turnitin catching substantially more AI content than Grammarly, which often misses lightly edited or paraphrased AI text entirely.

But accuracy isn't everything. Let's break down what each tool does, who it's built for, and when you should use one over the other.

AI Detection Accuracy: The Numbers

Turnitin claims 98% accuracy on fully AI-generated text, though independent testing suggests real-world accuracy is closer to 85-86% when accounting for edited and hybrid content. Turnitin's chief product officer has acknowledged they deliberately let some AI content pass to minimize false positives.

Grammarly's AI detection is notably less robust. It was added as a secondary feature to a writing assistant, not built as a dedicated detection system. User reports indicate it often underreports AI involvement and misses content that has been lightly edited or paraphrased.

Who Each Tool Is Built For

Turnitin is designed for academic institutions. It's the standard tool used by universities, colleges, and high schools worldwide. It provides access to massive databases of student papers, integrates with learning management systems (Canvas, Blackboard, Moodle), generates detailed similarity reports with source matching, and breaks down AI writing percentages by section.

Grammarly is built for individual writers and professionals. It's primarily a writing assistant with AI detection as a secondary feature. The platform handles grammar, spelling, and style corrections, offers tone adjustments, provides basic plagiarism checking against web content, and includes AI detection (added more recently).

The Grammarly-Turnitin Interaction Problem

Here's something many students don't realize: using Grammarly's AI features can trigger Turnitin's AI detection.

Basic Grammarly corrections (spelling, grammar, punctuation) won't cause problems. Turnitin isn't designed to flag simple edits.

However, Grammarly's advanced AI features (like GrammarlyGO and the paraphrasing tool) use the same underlying AI models that Turnitin is trained to detect. If you heavily rewrite content using Grammarly's AI suggestions, Turnitin may flag that content as AI-generated.

The rule of thumb: Use Grammarly for corrections, not for rewriting. If you need to rephrase AI content to avoid detection, you need a dedicated AI humanizer rather than a grammar tool.

Plagiarism Detection Comparison

For plagiarism (not AI) detection, the gap widens further.

Turnitin compares submissions against billions of web pages, over 70 million student papers, major academic publications and journals, and institutional databases. This comprehensive coverage makes it the gold standard for academic integrity.

Grammarly compares against public web pages and limited academic content via ProQuest, with no access to student paper databases. In academic contexts, Grammarly's plagiarism checker can miss critical overlaps that Turnitin would catch immediately.

Pricing Breakdown

Turnitin isn't available for individual purchase. It's sold exclusively to institutions. If you're a student, your school either has it or doesn't.

Grammarly offers individual plans, making it accessible to anyone, but its AI detection capabilities are limited compared to what institutions get with Turnitin.

When to Use Each Tool

Use Turnitin when you're an educator checking student submissions, your institution requires it for academic integrity, you need detailed AI writing reports with section breakdowns, or accuracy matters more than convenience.

Use Grammarly when you need grammar and style corrections, want a quick plagiarism check for business content, are working outside academic contexts, or need an accessible and affordable option.

Use neither for foolproof AI detection (no tool is 100% accurate), detecting heavily humanized AI content, or making final judgments without human review.

What About Beating AI Detection?

Both Turnitin and Grammarly can be fooled by well-humanized AI content. Their detection relies on patterns: sentence structure predictability, word choice consistency, and statistical markers that distinguish AI from human writing.

When AI content is properly humanized (with varied sentence lengths, natural phrasing, and authentic voice) detection accuracy drops significantly. Turnitin acknowledges this, estimating they catch about 85% of AI content while letting 15% pass to minimize false positives.

If you're using AI as a writing assistant and want your content to read naturally, running it through an AI humanizer before submission can transform detectable patterns into authentic-sounding prose. You can also use our free AI detector to check how your content scores before submission.

The Bottom Line

For academic AI detection: Turnitin wins decisively. It's more accurate, has better institutional integration, and provides detailed reports that educators need.

For individual writers: Grammarly is more accessible, but don't rely on its AI detection for anything high-stakes. It's best used as a writing assistant, not a detection tool.

For everyone: Remember that no AI detector is perfect. Both tools have false positives and false negatives. Treat detection scores as starting points for investigation, not definitive verdicts.