Grammarly AI Checker: Free Detector Review & Accuracy

Grammarly’s AI detector flags AI-generated text for free—but it also flags human writing at a 34% rate, and it cannot replace purpose-built tools like Turnitin for academic submissions. I tested the data to see where the free detector holds up and where it falls short.

Ranked: #1 Free AI Checker · Detected Models: ChatGPT, Gemini · Additional Tools: Plagiarism Checker, AI Humanizer · Integration: Google Docs, Microsoft Word

What is Grammarly AI checker?

Grammarly AI checker is a free web tool that scans text and flags sections it believes were generated by artificial intelligence. It sits within Grammarly’s broader ecosystem alongside grammar checking and plagiarism detection, which means users who already rely on Grammarly for writing assistance get AI detection at no extra cost.

Key features of Grammarly AI detector

The detector works directly in the browser—no desktop app required. Paste any text up to 10,000 characters and the tool returns a percentage score indicating how likely it is AI-generated. It also highlights specific sentences that triggered its judgment, letting writers see exactly where the flag was raised. The tool integrates with Google Docs and Microsoft Word through browser extensions, so documents can be checked without reformatting.

Supported AI models

Grammarly’s detection engine targets ChatGPT and Gemini outputs, according to the company. Third-party tests suggest it catches content from most mainstream large language models, though independent verification of model-by-model accuracy rates has not been published.

Is Grammarly AI checker free?

Yes, a free tier exists. Anyone with a Grammarly account can access the AI detector without paying. The catch is the 10,000-character limit per scan, which is enough for most essays or business documents but falls short for longer manuscripts or multiple submissions.

Free vs premium features

Grammarly Premium costs $12 per month and bundles AI detection with advanced grammar suggestions, tone adjustments, and plagiarism checking. For users who only need AI detection, the free version does the job—the premium upgrade adds value mainly for those who want a full writing assistant.

Usage limits

The free AI detector caps at 10,000 characters per check. QuillBot’s free AI detector is more restrictive at 1,200 words. Neither free tier removes the character limit by upgrading browser extensions alone—payment is required.

How accurate is Grammarly AI detector?

The accuracy picture is mixed. In Pangram’s 2026 test of 30 AI detectors, Grammarly flagged all 9 AI-generated samples correctly, resulting in a 0% pass rate for AI text. However, it also produced a 34% false positive rate, marking one in three human-written samples as AI. That trade-off matters depending on what you’re using it for.

User reports from Reddit

Reddit discussions reveal frustration from users whose natural writing triggered AI flags. A recurring complaint is that Grammarly over-suggests changes and flags phrasing that sounds formal or structured, even when a human wrote it. This aligns with the 34% false positive figure reported by Supwriter’s February 2026 tests on 200 essays.

Integration with docs

The Google Docs and Microsoft Word integrations mean Grammarly can check documents on the fly. Users report that the inline highlights make it easy to identify flagged passages, though the browser-based version lacks batch processing for multiple files.

“Neither Grammarly nor QuillBot was designed to bypass AI detectors, and their results reflect that.”

— Supwriter (Reviewer)

Grammarly AI checker vs competitors

Purpose-built detectors like Turnitin consistently outperform Grammarly in the accuracy stakes, but they serve different audiences. Turnitin targets universities with licensed access; Grammarly targets individual writers and small teams. Winston AI and QuillBot occupy the mid-range, with QuillBot’s paraphrasing tool adding complexity when evaluating both products.

Vs Turnitin AI checker

Turnitin remains the gold standard for academic AI detection. It reports a false positive rate under 1% for texts over 300 words, compared to Grammarly’s 34%. In Plagaireport’s analysis, Turnitin achieved 98% accuracy on humanized text, while Grammarly relies on a detection engine that was not designed for that task. Turnitin updated its algorithms in late 2025 specifically to catch QuillBot paraphrasing patterns, raising the bar for all competitors.

Vs Winston AI, QuillBot

QuillBot’s free AI detector let 44% of AI-generated text pass in Pangram’s tests—significantly worse than Grammarly’s 0% pass rate. However, QuillBot’s paraphrasing tool complicates the picture: it can humanize AI text well enough that other detectors flag it. Quillbot paraphrased ChatGPT content scored 100% AI on Turnitin in 2026 video tests, meaning QuillBot’s humanizer cannot reliably fool purpose-built detectors. Winston AI appears in top-detector lists but specific accuracy comparisons are scarce, with GPTZero noting limited data.

“Turnitin says its AI detector is fairly reliable, as long as the text is long enough: for submissions over 300 words, it reports a false positive rate of under 1%.” For those interested in kitchen appliances, you can find a Sunbeam Air Fryer Review here: $Sunbeam Air Fryer Review.

— GPTZero (AI Detector Provider)

The comparison data shows a wide gap: purpose-built detectors catch AI and humanized text at high rates, while Grammarly’s engine works best for pure AI output but produces false positives on human writing.

Turnitin’s licensed model means it is not directly comparable to free tools, but its accuracy benchmarks illustrate where Grammarly and QuillBot fall short for academic verification.

What is Grammarly AI humanizer?

Grammarly AI humanizer is a feature within the Grammarly ecosystem that rewrites AI-detected passages to sound more natural. It targets text the detector flagged, rephrases sentence structure and word choice, and adds citations support. The goal is to produce a version that passes re-scanning as human-written.

How it humanizes AI text

The humanizer runs after detection identifies flagged sections. It rephrases flagged passages using synonym changes, sentence restructuring, and tone adjustments. BrowserAct reports Grammarly Premium offers a 96% success rate in AI bypass strategies, though these figures come from promotional materials and have not been independently verified at scale.

Rewrite flagged content

Users paste flagged text, apply the humanizer, then run detection again to check whether the rewrites cleared the AI flag. The process works in rounds—detect, rewrite, detect again—until the score drops. QuillBot’s Creative mode produces similar results but with a known downside: it can generate awkward phrasing that sounds more robotic than the original.

For students submitting essays, the choice is straightforward: Turnitin is the authoritative detector and Grammarly cannot replace it for academic verification. For freelancers, marketers, or casual users who want to check whether their own writing reads as AI, Grammarly’s free tier delivers instant results without a subscription. The 34% false positive rate means you should treat flagged passages as suggestions to review, not as definitive AI verdicts.