How Does Grammarly Detect AI? What It Actually Checks

What Grammarly actually is

Grammarly is best known as a writing assistant, with AI detection added as part of its broader feature set. Understanding how it works matters because a flagged result can affect a grade, a job application, or a published piece, and the best defense is knowing what the tool actually measures before you rely on its score.

Grammarly's detector sits inside its existing writing tool, analyzing text for the likelihood it was AI-generated alongside its usual grammar and tone suggestions. Because users often draft and check in the same place, the detection is woven into the writing flow rather than a separate step.

The advantage of in-flow detection is that you catch problems while the text is still easy to change. A flag during drafting costs seconds to fix; the same flag discovered after submission is far harder to address.

What Grammarly checks and where it struggles

The signals these detectors watch for

• Phrasing patterns common in large language model output.
• Low variation in sentence structure and length.
• A tone that reads as generic or overly polished.
• Predictable vocabulary and transition choices.
• A polished, generic tone that could have been written about any topic, with no trace of a specific writer behind it.

How detection scores behave

How to keep your writing clear of flags

1. 1
   Write the substance yourself

   Use AI for research and outlining, then write the actual sentences. A detector scores the prose, so the words need to come from you.

2. 2
   Vary your sentence length on purpose

   Mix short and long sentences to add the variation detectors associate with human writing. Read a paragraph aloud to check the rhythm.

3. 3
   Add concrete details

   Insert specific examples, numbers, and named sources. Specifics are the strongest signal that a person wrote the text.

4. 4
   Run a check before you submit

   Use a detector on your own draft to find which sections still read as machine-generated, then rewrite those specifically.

A practical takeaway

Use Grammarly's detection as a checkpoint while you draft, then confirm with a dedicated detector before high-stakes submissions. The combination catches more than either tool alone.

For a quick self-check, run your draft through the free detector at AI Humanizer Lab alongside Grammarly. If a section flags on either, the humanizer tool can help smooth it, with no signup and no word limit, before you submit or publish.

Keep your perspective on what these scores mean. Grammarly is a useful signal, but it is one input among several, and the most reliable test of whether your writing is genuinely yours is still whether you can stand behind every sentence it contains.