AI-generated writing is now everywhere — in essays, blog posts, emails and job applications. That is why so many people search for the Grammarly AI detector: they want a quick way to check whether a piece of text was written by a person or by an AI tool such as ChatGPT. In this guide, we explain what Grammarly’s AI detector does, how these tools work, how accurate they really are, and which free options are worth trying.
What Is the Grammarly AI Detector?
The Grammarly AI detector is a free online tool from Grammarly that estimates how much of a piece of text appears to have been written by AI. You paste your text — or upload a document — and the tool returns a percentage score, along with an indication of which parts resemble AI writing.
It sits alongside Grammarly’s wider writing tools for grammar, spelling, tone and plagiarism. Before you submit an essay or publish a post, you can quickly check whether your writing sounds natural and human, or carries the patterns typical of AI text.
Grammarly says its detector can identify text from ChatGPT, Gemini, Claude and Microsoft Copilot. At the time of writing, the basic detector is free on its website; Pro subscribers get a more detailed agent that highlights specific passages and offers rewrites.
For a broader look at how AI is changing the way we write and work, see our overview of artificial intelligence in 2026.
How Does the Grammarly AI Detector Work?
AI detectors do not read like a teacher or editor. They use machine learning models trained on large collections of human-written and AI-generated text, looking for statistical patterns that tend to separate the two.
Grammarly explains that its detector analyses several characteristics:
- Sentence structure and predictability. AI text often follows very regular, predictable patterns, while human writing varies more in sentence length and rhythm.
- Repetition and uniformity. AI models frequently repeat phrases or reuse the same sentence shapes; human writers vary more, with quirks and uneven pacing.
- Language patterns. Certain word choices are more common in AI output: generic transitions, overly balanced sentences and cautious, neutral wording.
- Section-by-section analysis. Grammarly analyses each section separately, then combines the results into an overall percentage score.
A result of, say, 60% does not prove that 60% of the words were typed by a chatbot. It is an estimate — the model’s best judgement about how closely the writing resembles the AI-generated text it was trained on — and it is why no detector should ever be treated as proof on its own.
How to Use Grammarly’s Free AI Detector
Using the tool takes less than a minute. Here is the process, as described on Grammarly’s own website:
- Go to the detector page. Open Grammarly’s AI detector page in your browser.
- Paste your text or upload a document. Copy the text you want to check into the box, or upload a file.
- Click “Scan for AI”. Within seconds, you will see a percentage score for how much of your text appears to be AI-generated.
- Review the flagged sections. Consider rewriting highlighted passages in your own voice, with personal examples or a more natural tone.
Tip: check longer pieces in sections. Short texts give detectors little to work with, so a single paragraph gives far less reliable results than a full page.
How Accurate Are AI Detectors?
This is the question most people really want answered — and the honest answer is that no AI detector is 100% accurate. Grammarly itself states this plainly on its AI detector page: detectors can indicate characteristics found in AI-generated writing, but they cannot definitively conclude whether AI was used.
Grammarly does make a specific claim: at the time of writing, it states its detector achieves 99% detection accuracy and ranks first on RAID, an independent benchmark. Benchmarks are measured under controlled conditions, though — real-world writing is messier — so treat any accuracy claim as a guide, not a guarantee.
The wider industry picture supports caution. OpenAI, the company behind ChatGPT, launched its own AI text classifier in early 2023 and later withdrew it, noting its limitations. You can read OpenAI’s original announcement in its blog post on its AI text classifier. The lesson is clear: even the companies building these models find reliable detection genuinely difficult.
Limitations and False Positives
Detectors make two kinds of mistake:
- False positives: human-written text flagged as AI. This happens most often with formal or very polished writing — legal text, academic abstracts, technical documents and essays by non-native English speakers can all trigger detectors, because clear, structured writing resembles AI output.
- False negatives: AI-generated text that passes as human. Lightly edited AI text, paraphrased text, or output from newer models is much harder to catch, because editing breaks up the very patterns detectors rely on.
Two further limits: detection is a moving target, as AI models keep producing more human-like text; and short or mixed text is unreliable — a score on a short paragraph, or a document blending human and AI writing, deserves extra caution.
A detector result should therefore start a conversation about a piece of writing, not end it — especially in schools and workplaces, where a wrong result can have real consequences.
Best Free AI Detectors to Try
Grammarly is a good starting point, but it is not the only free AI detector available. Comparing results across two or three tools usually gives a more balanced picture than trusting any single score. Free tiers and limits change often, so check each tool’s own website for current details at the time of writing.
1. Grammarly AI Detector
The most convenient choice if you already use Grammarly. Free on the web, it gives a clear percentage score and connects with Grammarly’s grammar, plagiarism and rewriting tools.
2. GPTZero
One of the earliest dedicated AI detectors, built for students and educators. It highlights individual sentences that look AI-generated.
3. ZeroGPT
A straightforward free web tool that returns a percentage score, highlights sections it considers AI-written, and also offers plagiarism checks.
4. QuillBot AI Detector
QuillBot, best known as a paraphrasing tool, also offers a free AI detector — handy if you already use it for rewriting.
5. Copyleaks AI Detector
A plagiarism and AI detection platform used by schools and businesses, with a free web checker for text and files.
If you want to explore more free tools beyond detectors, our round-up of the best free AI tools in 2026 covers writing assistants, image generators and more.
Practical Tips for Checking AI-Written Text
These habits will help you get more from any free AI detector:
- Use more than one detector. Run the same text through Grammarly and one or two other free tools. If they broadly agree, the result is more meaningful than a single score.
- Check long passages, not single sentences. Detectors need a decent sample — a few hundred words at least — before their scores mean much.
- Look for the signs yourself. AI text often has tell-tale qualities: very even paragraph lengths, generic examples and confident statements without sources. Human writing usually has rough edges — opinions, specific details and uneven rhythm.
- Ask for the writing process, not just the result. Drafts, notes and edit history tell you far more about authorship than any detector score.
- Do not rely on detection alone. Combine detector results with plagiarism checks and your own reading. Remember that using AI as a brainstorming aid is not the same as submitting untouched AI output.
If you produce content with AI help yourself, our complete guide to AI content generators explains how to use these tools well while keeping your writing original and human-sounding.
FAQs
Q: Is the Grammarly AI detector free?
A: Yes. At the time of writing, the basic detector is free on Grammarly’s website — paste your text, click “Scan for AI” and you get a percentage score within seconds. Pro subscribers get a more detailed agent with passage-level explanations and rewrites.
Q: How accurate are AI detectors?
A: No AI detector is 100% accurate. Grammarly states 99% accuracy on the RAID independent benchmark, but real-world results vary with text length, editing and the model used. Detectors estimate likelihood; they cannot prove authorship.
Q: Can AI detectors tell ChatGPT text apart from human writing?
A: Usually, yes — detectors are trained on ChatGPT output and can often recognise its patterns. But heavily edited, paraphrased or very short ChatGPT text is much harder to detect, and human writing can also be wrongly flagged. A detector result is an indication, not a certainty.
Q: Do AI detectors work on paraphrased text?
A: Poorly. Paraphrasing — whether done by a person or a tool — breaks up the predictable patterns detectors look for, so AI text that has been rewritten often passes as human. This is one of the biggest limitations of current detection tools.
Q: Was this written by AI? Can a detector answer that question?
A: Not definitively. A detector can only tell you whether text resembles AI-generated writing. If the question matters — an exam, a job application, a published article — combine detector results with plagiarism checks, draft history and a conversation with the writer.
Conclusion
The Grammarly AI detector is one of the easiest free AI detectors to use: fast, free, and with a clear percentage score. Tools such as GPTZero, ZeroGPT, QuillBot and Copyleaks offer useful second opinions.
But remember what Grammarly and OpenAI both say: no detector can conclusively prove whether AI wrote a piece of text. False positives happen, paraphrasing defeats most detectors, and short passages are unreliable. Use these tools as a first check, combine them with plagiarism checks and your own reading, and treat the writer — not the score — as the final authority.
