Key takeaways
- Grammarly's AI detector is free and gives a percentage estimate, not proof.
- Grammarly claims 99% accuracy but also warns that human writing can be flagged.
- Short, formal and non-native English writing are flagged more often.
- Its score won't match Turnitin's; check with Turnitin if that's what your school uses.
Grammarly's AI detector is free, fast and built into a tool millions of people already use, so it's often the first check students run. But a percentage on a screen feels more certain than it is. Here's what the score means, what Grammarly itself says about accuracy, and what to do with the result.
What Grammarly's AI detector does
You paste your text and Grammarly estimates what percentage of it looks AI-generated. It splits the text into sections and looks for language patterns often linked to AI writing, like predictable sentence structure and repetition. The basic checker is free on Grammarly's website.
So, is it accurate?
Grammarly says its detector reaches 99% detection accuracy and ranks first on RAID, an independent research benchmark for AI detectors. On the same page, it also warns that no AI detector is 100% accurate, that even human writing can be flagged as AI, and that a detector's result should never be relied on alone.
Both things are true. A detector can score well on a benchmark and still be wrong about your essay, because a benchmark measures an average over many texts, and you only care about one.
Why human writing gets flagged
Detectors don't know who wrote a text. They measure how predictable it is, and some human writing is very predictable:
- Short texts. With only a few sentences there isn't enough signal. Turnitin, for example, needs about 300 words of prose before it gives an AI score at all.
- Formal or formulaic writing. Lab reports, summaries and five-paragraph essays follow patterns that look machine-made.
- Non-native English. A 2023 Stanford study found that popular GPT detectors flagged more than half of essays written by non-native English speakers as AI-generated.
- Heavy editing tools. Text that has been polished sentence by sentence by any AI tool can pick up the same patterns.
Grammarly's score won't match Turnitin's
Every detector uses its own model, so the same essay can score very differently on Grammarly, GPTZero and Turnitin. If your school uses Turnitin, Grammarly's score is only a rough hint of what your instructor will see. Turnitin also hides AI scores between 1% and 19% behind an asterisk, because its own testing found more false positives at low scores.
If the Turnitin result is the one that matters, check with Turnitin itself. You can run a real Turnitin check and get both reports before you submit.
What to do with your score
- Low score: you're probably fine. Keep your drafts and notes in case anyone asks how you wrote it.
- High score on text you wrote: don't panic. Vary your sentence length, add your own examples, and say things the way you'd say them out loud. Then check again.
- High score on an AI-assisted draft: rewrite it in your own voice, or run it through a humanizer like Unbot AI, read the result, and check the score again. Follow your school's rules on AI use.
- Always: check with more than one detector. When two disagree, neither is proof.