Does Grammarly Detect AI Writing? We Tested It

The short answer: barely.

We ran 500 texts through Grammarly's AI detector. The results were... not great. Out of 250 human-written samples, Grammarly flagged 85 of them as AI-generated. That's a 34% false positive rate. And out of 250 confirmed AI-generated texts, it only caught 120. Meaning it missed 52% of actual AI content.

If you're a student worried about getting falsely flagged, a writer trying to verify your own work, or a teacher trying to catch AI submissions, those numbers should concern you. We spent three weeks running this test because we kept seeing people ask whether Grammarly's built-in detector is good enough. We wanted a real answer, not marketing copy.

Here's everything we found.

TL;DR: Grammarly's AI Detector Isn't Reliable

Quick summary of our findings:

• 34% false positive rate — flagged human-written text as AI over a third of the time
• 52% miss rate on AI text — more than half of AI-generated content slipped through undetected
• Only catches obvious ChatGPT output — struggles with Claude, Gemini, and any text that's been lightly edited
• High confidence on wrong calls — often reported 90%+ AI probability on fully human text

Bottom line: if you're relying on Grammarly alone to detect AI writing, you're going to get burned. Dedicated tools like GPTZero and Originality.ai perform significantly better, and SupWriter can humanize AI text to bypass even those.

How We Tested Grammarly's AI Detection

We didn't want to run a quick test with ten paragraphs and call it a day. We built a proper dataset. Here's what it looked like:

The Dataset

250 human-written samples collected from published authors, student essays (with permission), professional blog posts, and personal emails. We deliberately included a mix of writing quality — from polished magazine articles to rough first drafts with typos.

250 AI-generated samples produced by ChatGPT (GPT-4o), Claude 3.5 Sonnet, and Gemini 1.5 Pro. We split these roughly evenly across the three models. Each sample was generated with a single prompt and no manual editing, so we were testing raw AI output.

Content Types

We tested four categories of content:

• Academic essays (argumentative, analytical, and research-style)
• Blog posts (tech, marketing, lifestyle, how-to guides)
• Professional emails (cold outreach, internal communication, follow-ups)
• Creative writing (short fiction, personal narratives, opinion pieces)

Every sample was between 300 and 1,500 words. We ran each one through Grammarly's premium AI detection feature and recorded three things: the AI probability score, the binary verdict (AI or human), and the confidence level.

What We Controlled For

We used fresh Grammarly Premium accounts to avoid any bias from account history. Each text was pasted directly into the Grammarly editor. We tested over a two-week window in February 2026 to account for any backend updates. And yes, we ran duplicates to confirm consistency — the scores were stable within about 3 percentage points on repeated scans.

The Results: Grammarly's AI Detector Performance

Let's start with the topline numbers, then dig into the breakdown.

Overall Accuracy

Grammarly correctly identified the source (human or AI) in 57% of all samples. For context, random chance would give you 50%. So Grammarly's detector is only marginally better than flipping a coin.

Honestly, this surprised us. We expected mediocre performance, not near-random performance.

False Positive Rate by Content Type

The false positive rate (human text incorrectly flagged as AI) varied wildly depending on what kind of writing we tested:

The academic essay numbers are particularly concerning. A 41% false positive rate means that if a teacher uses Grammarly to check a stack of student papers, nearly half the legitimate ones could get flagged. That's not a tool you can make decisions with.

On the flip side, the 61% miss rate on professional emails means Grammarly lets most AI-written emails through without a peep. We think this is because email writing tends to be formulaic regardless of whether a human or AI wrote it.

Performance by AI Model

We also broke the results down by which AI model generated the text:

• ChatGPT (GPT-4o): Grammarly caught 58% of outputs. This was the best detection rate, likely because Grammarly's training data leans heavily on earlier GPT models.
• Claude 3.5 Sonnet: Only 41% caught. Claude's writing style tends to be more varied and less pattern-heavy, which clearly trips up the detector.
• Gemini 1.5 Pro: 39% caught. Gemini flew under Grammarly's radar almost as easily as Claude.

How Grammarly Compares to Dedicated Detectors

We ran the same 500-sample dataset through three popular dedicated AI detectors. The difference was stark:

Originality.ai came out on top overall. Turnitin had the lowest false positive rate, which makes sense given how catastrophic false accusations are in academic settings. GPTZero struck a decent balance. And Grammarly trailed behind all of them by a wide margin.

The gap isn't subtle. We're talking about a 32-percentage-point accuracy difference between Grammarly and Originality.ai. These are fundamentally different tiers of detection quality.

Why Grammarly's AI Detector Falls Short

Grammarly is a phenomenal grammar checker. Genuinely. We use it ourselves. But being great at grammar correction doesn't translate to being great at AI detection. Here's why the detector underperforms:

It's a Grammar Checker First, Detector Second

Grammarly added AI detection as a feature, not as a core product. Their engineering team has spent over a decade optimizing for grammar, clarity, and tone suggestions. The AI detector was bolted on in response to market demand. It uses a classifier built on top of their existing language models, not a purpose-built detection architecture.

Compare that to GPTZero or Originality.ai, where AI detection is the entire product. Their teams wake up every morning thinking about one thing: catching AI-generated text. That focus shows in the results.

Not Trained on the Latest AI Models

AI models evolve fast. GPT-4o writes differently than GPT-3.5. Claude 3.5 writes differently than Claude 2. Grammarly's detector appears to be primarily calibrated against older GPT-style outputs. That explains why it caught 58% of ChatGPT text but only 39-41% of Claude and Gemini output.

Dedicated detectors update their models regularly to keep pace with new AI releases. Grammarly's update cycle for detection seems slower, which is understandable — it's not their primary business.

Struggles with Rewritten or Edited AI Text

We ran a small follow-up test. We took 50 AI-generated samples and made minor edits: swapping a few words, adjusting sentence structure, breaking up long paragraphs. Nothing aggressive. Maybe five minutes of light editing per piece.

Grammarly's detection rate on these edited samples dropped from 48% to just 19%. Meanwhile, Originality.ai still caught 72% of the edited samples. This tells us Grammarly's detector relies heavily on surface-level patterns that even basic editing disrupts.

High Confidence Scores on Wrong Predictions

This was the most frustrating finding. When Grammarly got it wrong, it wasn't uncertain about it. We saw human-written academic essays flagged with 92% AI probability. We saw raw ChatGPT output cleared with an 88% human probability score.

A detector that's wrong 43% of the time is bad enough. A detector that's wrong 43% of the time and confident about it is dangerous. It gives users a false sense of certainty in either direction.

What Actually Works for Detecting (and Humanizing) AI Text

Depending on which side of the detection question you're on, here are our recommendations based on the data:

If You Need to Detect AI Writing

Skip Grammarly for detection. Use a dedicated tool. Based on our testing, here's the ranking:

1. Originality.ai — Best overall accuracy (89%), lowest false positive rate among paid tools. Costs $14.95/month for 2,000 scans.
2. Turnitin — Best for academic contexts with just a 3% false positive rate. Only available through institutional licenses.
3. GPTZero — Strong free tier with 84% accuracy. Good starting point if you don't want to pay.

We covered this in more depth in our best AI detector tools comparison. That guide includes pricing, methodology, and edge-case testing we didn't cover here.

One important caveat: no AI detector is 100% accurate. Even the best tools make mistakes. We strongly recommend against using any single detector result as the sole basis for an accusation of AI use, especially in academic or professional contexts.

If You Need AI Text to Sound Human

Maybe you're on the other side of this. You use AI to draft content, and you need it to read naturally and pass detection checks. That's where SupWriter comes in.

We built SupWriter specifically to humanize AI-generated text. Not by spinning words or adding random errors, but by restructuring sentences, varying rhythm, and introducing the kind of natural imperfections that characterize real human writing.

Our internal testing shows a 99%+ bypass rate against GPTZero, Turnitin, Originality.ai, and yes, Grammarly's detector (though that one wasn't exactly hard to beat). The output doesn't just fool detectors — it genuinely reads better. More natural, more varied, more like something a person actually sat down and wrote.

If you're currently using Grammarly and wondering about alternatives that handle both grammar correction and AI humanization, check out our Grammarly alternative comparison.

Frequently Asked Questions

Does Grammarly have a built-in AI detector?

Yes. Grammarly added an AI detection feature to its premium plan in 2024. It provides a percentage score estimating how likely a piece of text is to be AI-generated. However, based on our testing with 500 samples, the accuracy is significantly lower than dedicated AI detection tools. It works as a rough indicator but shouldn't be relied on for definitive conclusions.

Can Grammarly detect ChatGPT writing specifically?

Grammarly performs best against ChatGPT output compared to other AI models, catching about 58% of raw ChatGPT text in our tests. But that still means 42% of ChatGPT writing slips through. For Claude and Gemini output, the miss rate climbs to 59-61%. If someone lightly edits the ChatGPT output before running it through Grammarly, detection drops to around 19%.

Is Grammarly's AI detection free?

No. AI detection is a Grammarly Premium feature. The free version of Grammarly only covers basic grammar and spelling checks. You'll need a Premium subscription (starting at $12/month billed annually) to access the AI detection functionality. Given the accuracy numbers we've shared, you might want to consider whether a dedicated detector offers better value for that specific need.

Why does Grammarly flag my writing as AI when I wrote it myself?

This is the false positive problem, and it happens more often than you'd expect. In our testing, Grammarly falsely flagged 34% of human-written text. Academic writing and creative writing were hit hardest (41% and 44% false positive rates respectively). This tends to happen with formal, structured writing or when you naturally write in a clear, organized style. Ironically, "good" writing gets flagged more often because it shares surface-level traits with AI output — clean grammar, logical flow, consistent tone.

What's the most accurate AI writing detector in 2026?

Based on our 500-sample benchmark, Originality.ai leads with 89% overall accuracy and an 8% false positive rate. Turnitin follows with 86% accuracy and the lowest false positive rate at 3%, making it the safest choice for academic settings. GPTZero offers solid 84% accuracy with a generous free tier. Grammarly sits well behind at 57%. For a complete breakdown with pricing and methodology, see our AI detector tools comparison. Keep in mind that all detectors have limitations and none should be treated as infallible.

The Bottom Line

Grammarly is an excellent writing assistant. We use it. We recommend it for grammar and clarity. But its AI detector is not something we can recommend to anyone who needs reliable results.

A 34% false positive rate means real consequences for real people. Students accused of cheating. Writers questioned about their authenticity. Professionals losing credibility over a bad algorithm call. That's not acceptable when better tools exist.

If you need to detect AI text, use a purpose-built detector. If you need AI text to read like a human wrote it, give SupWriter a shot. Either way, don't rely on Grammarly for this particular job. It's like using a Swiss Army knife to fell a tree — it technically has a blade, but you're going to have a bad time.

Related Resources

• Best Grammarly Alternative — For AI humanization
• Grammarly vs QuillBot 2026 — Head-to-head comparison
• Grammarly vs ChatGPT — Which creates more detectable text?
• Grammarly for Students — Best student alternative
• Best AI Humanizer Tools — Complete ranked list
• QuillBot Review 2026 — Honest test results