The short answer
Not reliably enough to use alone. Grammarly says its AI detector reaches 99% accuracy and ranks first on the RAID benchmark, but one small independent test of the free version caught none of nine AI samples. Plagino has not benchmarked Grammarly. Treat its score as a prompt to reread, not proof.
How accurate is Grammarly's AI detector?
Nobody outside Grammarly has published a large, recent test of its AI detector that we could verify, so the honest answer is that its accuracy is unproven for student essays. What exists is a strong vendor claim and a small independent test that points the other way.
Grammarly's own AI detector page says the tool "achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark". The same page also says that "no AI detector is 100% accurate" and that AI detection "should never be used as a standalone verification method". Its user guide adds that the percentage "should not be used as an objective source of truth".
Plagino has not benchmarked Grammarly. Our March 2026 benchmark covered seven detectors, and Grammarly was not one of them. Everything below about Grammarly's accuracy comes from Grammarly itself or from third parties, and we say which is which.
What the RAID ranking does and does not tell you
RAID is a research benchmark built by academic researchers and published at ACL 2024. It tests detectors on a very large set of generated text across many models, domains and attacks. In a February 2026 post, Grammarly said it ranked first on RAID's leaderboard for detection quality.
That is a meaningful result, but it is a leaderboard entry, not a study of student writing. The RAID paper itself warns that detectors are "easily fooled" by adversarial attacks, changes in sampling and unseen models. It also reports accuracy with the false positive rate fixed at 5%, which is a research convention and not necessarily the threshold a product uses by default.
What independent testing found
In a January 2026 comparison, Pangram Labs ran the free version of Grammarly's detector on nine AI-written samples (from GPT-4o, Gemini 2.0 and Claude 3.7 Sonnet) and three human texts. Grammarly passed all three human texts but flagged none of the nine AI samples at Pangram's 75% threshold.
Read that result with two caveats. Pangram sells a competing detector, and twelve samples is far too few to estimate a rate. It does show that a high benchmark rank does not guarantee the free tool will flag a polished AI essay.
How do Plagino and Grammarly compare side by side?
The two tools are built for different jobs. Grammarly is a writing assistant that added AI detection and plagiarism checking. Plagino is a pre-submission check that returns one report for a finished draft. The table shows what each one does as of October 2026.
| Plagino | Grammarly | |
|---|---|---|
| AI docs caught | 97.4% in Plagino's March 2026 benchmark (n=500) | Not tested by Plagino. Grammarly claims 99% accuracy |
| Human docs wrongly flagged | 0.8% in the same benchmark (n=500) | Not tested by Plagino. No false positive rate published |
| Price | Free: 3 scans/month. Pro: $19/month | Free AI detector. Pro: $12/month billed annually, $30 month to month |
| Plagiarism check | Yes, against the CORE open-access research index | Pro only, against web pages and ProQuest academic databases |
| Input | Upload a .docx file | Paste text, upload a file, or use it inside Google Docs and Word |
| Output | PDF report with sentence-level AI highlights and matched sources | Percentage score; Pro adds an AI Detector agent that points to flagged phrases |
| Writing-process record | No | Yes, through Grammarly Authorship |
Detection rate and false positives: what the numbers say
A detector can fail in two ways: it can miss AI writing, or it can flag a human. For a student, the second failure is the one that leads to an integrity meeting, so we report both separately.
Detection rate
In Plagino's benchmark, run in March 2026 on 500 AI-written documents from GPT-4o, Claude and Gemini (some lightly edited by people), Plagino flagged 97.4% (95% confidence interval roughly 95.6% to 98.5%). Turnitin did better on detection, at 98.2%. We publish that loss alongside our wins.
For Grammarly, the only detection figure is the vendor's: 99% accuracy, linked to its RAID ranking. Grammarly does not say on its detector page how that figure was measured on essays, and the FAQ on the same page says its model "was trained on tens of thousands of texts, including both human-written and AI-generated text created before 2021". It does not say how the model has been updated for newer chatbots beyond stating that it "updates continuously".
False positive rate
On 500 human-written documents in the same run, Plagino wrongly flagged 0.8% (interval roughly 0.3% to 2.0%). That was the lowest of the seven tools tested. Turnitin flagged 1.4%.
Grammarly does not publish a false positive rate. Its detector page says it is "designed to avoid wrongly flagging human-written text", and its user guide admits human writing can be flagged when its "language patterns, syntax, and complexity mirror those of AI-generated text". That matters most for non-native English writers: a 2023 Stanford study found that several AI detectors misclassified more than half of the TOEFL essays it tested as AI-generated. Our guide to AI detector false positives explains why.
What does each tool cost and what does it check against?
As of 3 October 2026, Grammarly's plans page lists three tiers. Free costs nothing but does not include the plagiarism check or the in-editor AI detection. Pro is $12 a month billed annually ($144 a year), $60 a quarter, or $30 month to month, and adds "Detect plagiarism and AI generated text". Enterprise is priced by quote. The stand-alone AI detector on Grammarly's website is free to use.
Grammarly's plagiarism checker compares text with "over 16 billion web pages and academic papers in ProQuest's databases". That is a broader source pool than Plagino's for web content, and it is a real advantage if your worry is copied web pages.
Plagino is free for 3 scans a month with no card, and Pro costs $19 a month for 20 scans a day. Its plagiarism check matches passages against open-access academic literature through CORE. It does not search the open web, paywalled journals or other students' papers, and it cannot see Turnitin's private repository. Each match it reports points at the exact overlapping words.
Privacy and data use
Grammarly is now part of Superhuman, and the Superhuman privacy policy (effective 6 July 2026) says the company does not make money "by selling or monetizing user content" and that you can decide whether your content trains its AI models through a training control in your account settings. The policy does not give a fixed retention period for user content. Grammarly's plagiarism page says checked text "will never be publicly searchable or stored in any database accessible by others".
Plagino reports go to your own account. A Plagino report, like any detector result, is a prompt to review the writing, not evidence of misconduct.
Where Grammarly is the better choice
Grammarly is the better tool in several common situations, and we would point you to it in these cases.
If you already pay for Grammarly Pro, its plagiarism and AI checks cost you nothing extra and run inside Google Docs and Word while you write. Plagino needs a finished .docx upload.
If you need a web plagiarism check, Grammarly searches billions of web pages and ProQuest. Plagino searches open-access research only, so it will not catch a paragraph lifted from a blog or news site.
If you want a record of how you wrote something, Grammarly Authorship sorts your text by where it came from, such as typed by you or pasted from an AI tool. A process record like that can be stronger evidence of your own work than any detector score, which our guide on being falsely accused of using AI covers in more detail.
And if you mainly want grammar, tone and clarity help, Grammarly is a writing assistant first. Plagino does not edit your writing.
Where Plagino is the better choice
Plagino fits best when you want one report on a finished draft that shows both kinds of risk. You upload a Word file and get a PDF with sentence-level AI highlights and the academic sources your text overlaps with, which you can keep alongside your drafts.
It is also the only one of the two with a published, document-level false positive rate from a fixed test set. If you write in a second language or in a formal, formulaic style, a low and published false positive rate is worth more than a high claimed accuracy. You can try the AI detector on a draft for free.
Test conditions and limitations of this comparison
Plagino's figures come from one benchmark run in March 2026: 1,000 documents (500 AI-written, 500 human-written), each tool used through its own web app on default settings, scored on whether a document was flagged at all. The 95% intervals are Wilson intervals on 500 documents per rate. Results on your own writing will differ, and a different corpus could rank the tools differently. The full method is on our benchmark page.
Grammarly was not in that run, so this page compares a measured result with vendor claims. The two are not like for like. Grammarly's 99% figure is its own claim, and the RAID leaderboard uses a different corpus and scoring rule. Pangram's test used twelve samples and comes from a competitor. Prices and features were checked on 3 October 2026 and change often.
Comparing other tools? See Plagino vs Scribbr, Plagino vs Winston AI and Plagino vs Copyleaks.
Frequently asked questions
Is Grammarly's AI detector accurate?
Grammarly says its detector reaches 99% accuracy and ranks first on the RAID benchmark, but it does not publish a false positive rate. One small independent test of the free version by Pangram Labs flagged none of nine AI samples. Grammarly itself says no detector is 100% accurate and its score should not be used on its own.
Is the Grammarly AI checker free?
Yes. The AI detector on Grammarly's website is free to use. The plagiarism check and the AI Detector agent inside the Grammarly editor need Grammarly Pro, which costs $12 a month billed annually or $30 month to month as of October 2026.
Can Grammarly's AI detector be wrong about human writing?
Yes. Grammarly's own user guide says human writing can be flagged when its patterns, syntax and complexity resemble AI text. Formal, formulaic and second-language writing is most at risk with any detector.
Does Grammarly's AI score match Turnitin?
No. Grammarly uses its own in-house model, so its score can differ from Turnitin, GPTZero or Plagino on the same text. A low Grammarly score does not predict a low Turnitin score, and the reverse is also true.
Does Grammarly keep the text I check?
Grammarly says checked text will never be publicly searchable or stored in a database others can access. Its parent company's privacy policy says it does not sell user content and lets you control whether your content trains its AI models, but it does not state a fixed retention period.
Has Plagino tested Grammarly's AI detector?
Not yet. Grammarly was not part of Plagino's March 2026 benchmark, which covered Turnitin, Plagino, Originality.ai, Copyleaks, GPTZero, Academi.cx and TurnDetect. This page uses Grammarly's own claims and third-party tests instead, and labels them as such.