GPTZero vs Grammarly AI Detector: Which One Should You Actually Trust?

Grammarly's AI detector is free, GPTZero's starts at 9,99 EUR. The honest 2026 comparison on accuracy, features and cost.

Grammarly's AI detector is free and claims 99% detection accuracy citing the RAID academic benchmark. GPTZero is the one institutions actually buy, with LMS integration and multilingual detection, but as of September 2026 its pricing page no longer advertises a free tier at all.

If you want a free self-check, that difference decides it. If you are an institution, it does not.

Key Takeaways

• Grammarly's basic detector is free and outputs a percentage plus sentence-level highlighting. Pro adds a deeper analysis agent.
• GPTZero's public pricing starts at 9,99 EUR per month for Premium, 300,000 words monthly and a 50,000 character limit per scan, checked 12 September 2026.
• GPTZero Professional is 19,99 EUR per month, adding 500,000 words, 150,000 characters per scan, 250-file batch upload and LMS integration.
• GPTZero wins on institutional features. Canvas and Google Classroom integration, downloadable reports and paraphraser detection are things Grammarly does not sell.
• Neither publishes a false-positive rate next to its accuracy claim, which is the number that actually affects you.

The headline difference: what each one costs

Let's start with the thing most comparisons get wrong, because it changed recently.

Grammarly's AI detector is free. You get a score and sentence-level highlighting without paying. Grammarly Pro adds an "AI Detector agent" that goes beyond the score with deeper analysis and suggestions, but the detection itself is not paywalled.

GPTZero's pricing page, checked on 12 September 2026, shows two plans. Premium at 9,99 EUR per month billed annually gives you 300,000 words per month with a 50,000 character cap per individual scan. Professional at 19,99 EUR per month raises that to 500,000 words, 150,000 characters per scan, batch upload of up to 250 files and enterprise LMS integration.

What the comparison table does not show is a free column. GPTZero still offers a free entry point through its "Get started for free" flow, but it no longer sits in the published plan comparison, and the limits are not stated there. That is a meaningful change for anyone who found GPTZero through a search for a free checker, and it is why we track GPTZero's free tier limits separately.

Accuracy: two claims, neither complete

Grammarly claims 99% detection accuracy and a first-place ranking on RAID, a shared academic benchmark built specifically to test detector resilience against paraphrasing and adversarial edits. The RAID paper is real published research, which puts Grammarly's claim on firmer ground than most.

GPTZero publishes accuracy claims too, and markets heavily on institutional trust: over 10 million teachers and students, use in more than 100 countries, and a partnership with the American Federation of Teachers.

Here is what neither presents clearly next to the headline figure: the false-positive rate on the same test set.

That is the number that matters to you. Detection accuracy tells you how often a tool catches AI text. False-positive rate tells you how often it accuses a human. A detector tuned aggressively can score beautifully on the first number while doing real damage on the second, and you cannot tell from a marketing page which trade-off was made.

This is not a knock specific to these two. It is the industry norm, and it is why we publish both figures together from our own 600-sample benchmark across ChatGPT, Claude, Gemini, DeepSeek, Grok and LLaMA.

Features: this is where GPTZero pulls ahead

For an individual checking a draft, feature lists barely matter. For a department, they are the whole decision.

GPTZero sells things Grammarly does not. Multilingual AI detection. Downloadable AI reports you can attach to an integrity case. Sentence-level highlighting with mixed human and AI document handling. Paraphraser and AI-bypasser detection. Batch upload for 250 files at once. Custom LMS integration for Canvas and Google Classroom. Page-by-page scanning for long documents.

That is a procurement feature set, and it explains why GPTZero appears on campuses while Grammarly mostly appears in individual writing workflows. If you are an instructor trying to process a class set of submissions, batch upload and an exportable report are not luxuries.

Grammarly's advantage is different and narrower: the detector lives inside the editor you are already using. Zero extra steps. For a working writer, that convenience beats a feature matrix.

The conflict nobody likes discussing

Both companies sell generative writing features.

Grammarly has generative rewriting built in. GPTZero is a detection-first company, but it now sells grammar checking, writing feedback and citation generation alongside detection.

This matters because a vendor that both generates and detects text has an awkward incentive structure. It is not evidence of wrongdoing. It is a reason to verify anything consequential against a tool that has no stake in the writing side.

That is the practical case for a third opinion. Our free AI detector gives you professional-grade detection across every major model on a free account with free credits on signup, at 99.99% accuracy on text. The capabilities rivals meter or paywall are simply open, which makes it a clean tiebreaker when two tools disagree.

Which should you pick

Three straightforward cases.

You are a student or writer checking your own work. Use Grammarly's free detector if you already write in Grammarly. Verify anything that matters with a second independent tool, because a single score has no business being the only evidence about your writing.

You are an instructor working solo. GPTZero's free entry point plus the exportable report is the better fit, but understand the limits before you rely on it. The same caution applies that we raised in our review of the best AI detectors for teachers.

You are buying for a department. GPTZero Professional is the realistic option of the two, because Grammarly does not sell the LMS integration, batch processing and reporting that institutional workflows require. Budget for the false-positive conversation before you deploy, not after.

What both tools share

Neither can reliably catch paraphrased AI text. Neither is dependable below roughly 300 words. Neither can tell you which model wrote something. Neither should ever be the sole basis for an accusation.

Grammarly says this plainly on its own product page. That honesty is worth more than a percentage point of benchmark performance.

Checking the claims yourself

Both vendors publish enough that you can verify the important parts in about five minutes, and the numbers move often enough that it is worth doing.

GPTZero's pricing page is the authoritative source for limits and cost. We checked it on 12 September 2026 and found the comparison table starting at Premium with no free column. If you are reading this months later, check it again rather than trusting our figures, because GPTZero has revised this repeatedly.

Grammarly's own detector page states the free access, the 99% claim and the RAID ranking. Same advice applies.

What neither page gives you is the number that decides whether a detector is safe to act on: the false-positive rate on human writing, measured on the same set as the accuracy claim. The gap matters most for the people most likely to be harmed by it. Liang et al., 2023 found detectors misclassifying non-native English writing as AI-generated at sharply elevated rates, a finding that has held up across subsequent work and that no vendor benchmark headline reflects.

So the honest comparison framework is not "which is more accurate". It is:

What does it cost, and at what per-scan cap? Verifiable on both pricing pages.

What does it do? Feature lists are checkable. GPTZero sells institutional workflow, Grammarly sells convenience.

What does it get wrong, and on whose writing? Neither vendor answers this, so assume the answer is worse than the marketing implies and never let a single score be the only evidence.

That third question is the one that should decide an institutional purchase, and it is the one procurement processes almost never ask.

FAQ

Is Grammarly's AI detector free and GPTZero's not?

Grammarly's basic detector is free. GPTZero has a free entry point but no longer lists free limits in its published pricing comparison, where plans start at 9,99 EUR per month.

Which is more accurate, GPTZero or Grammarly?

Grammarly cites a first-place RAID benchmark ranking and 99% detection accuracy. GPTZero publishes its own claims. Neither publishes a matching false-positive rate, so a direct accuracy comparison is not possible from public data alone.

Does GPTZero integrate with Canvas?

Yes, on the Professional plan. Custom LMS integration for Canvas and Google Classroom is a paid feature.

Can either detect paraphrased AI writing?

GPTZero markets paraphraser and AI-bypasser detection. In practice, paraphrasing degrades every detector in this class, so treat those results with extra caution.

Which one do colleges use?

GPTZero appears on campuses far more often, though Turnitin remains the dominant institutional tool by a wide margin.

The short version

Grammarly is the better free self-check. GPTZero is the better institutional product. Neither should be the only voice in a decision about someone's academic record.

Get a free second opinion at GPTOne. Free credits on signup, no card required.

Meta description: Grammarly's AI detector is free, GPTZero's starts at 9,99 EUR. The honest 2026 comparison on accuracy, features and cost.