Is Grammarly Accurate? We Tested All 4 Tools on 28 Texts

Is Grammarly accurate? It depends on which Grammarly you mean. On our 28 text test, the Grammarly AI detector scored only 8 of 16 AI generated texts at 50% or higher and gave three of them a 0% "All clear" result, while never scoring a human text above 15%. The grammar checker, on the other hand, caught 25 of 26 planted errors, and the plagiarism checker found 99% copied text in a verbatim Wikipedia passage.

I've spent nearly 20 years in AI development and digital marketing, and Grammarly is the sixth AI detector I've run through the same 28 texts this year, after GPTZero, ZeroGPT, JustDone, Originality.ai and Solvely. This time I went further than the detector. I tested every accuracy claim people search for: the AI detector, Grammarly's own Humanizer against three detectors, the plagiarism checker with known copies, and the grammar checker with a seeded error sheet. We paid for Grammarly Pro ourselves and Grammarly had no part in the test.

My Main Points:

• The Grammarly AI detector averaged 49.8% on 16 AI texts and 2.0% on 12 human texts. Five AI texts scored 80% or higher, five scored under 20%, and three scored 0%.
• No human text was flagged. Ten of 12 human texts scored 0%, and the highest human score was 15% on an economics paper introduction.
• On the same 16 AI texts, GPTZero, Solvely and Originality.ai scored 16 of 16 at 50% or more, ZeroGPT scored 10, and Grammarly scored 8.
• Grammarly's Humanizer beat Grammarly's detector 4 of 4 times, taking texts from 97% to 99% down to 0%. GPTZero still caught all four at 100% and Originality.ai at 88% to 92%.
• The plagiarism checker scored a verbatim Wikipedia copy at 99%, a verbatim Python docs copy at 92%, a lightly reworded copy at 3% and an original text at 0%.
• The grammar checker caught 25 of 26 planted errors, missed one tense shift, and flagged 2 of 10 correct sentences on style grounds.

In this post I'll show the detector results by text type, how Grammarly compares with GPTZero, ZeroGPT and Solvely on the same samples, what its Humanizer does to those scores, and where the plagiarism and grammar checkers stand. The samples and method are the same ones behind our AI detector accuracy hub.

Is Grammarly Accurate? All Four Tests at a Glance

Grammarly is accurate as a grammar checker and a copy and paste plagiarism checker, and weak as an AI detector. The table below sums up all four tests we ran on October 31, 2026 in the Grammarly Pro editor.

Grammarly tool	What we did	Result	Verdict
AI Detector	28 texts: 16 AI, 12 human	8 of 16 AI texts at 50% or more; 0 of 12 human texts above 15%	Safe for human writers, misses half of AI text
Humanizer	4 AI texts rewritten, rescanned by 4 detectors	Grammarly 0% on all 4; ZeroGPT 0% to 26.4%; GPTZero 100% on all 4; Originality 88% to 92%	Beats Grammarly and ZeroGPT, not GPTZero or Originality
Plagiarism Checker	2 verbatim copies, 1 reworded copy, 1 original	99%, 92%, 3%, 0%	Catches copy and paste, not paraphrase
Grammar Checker	365 words with 26 planted errors and 10 traps	25 of 26 caught, 2 of 10 traps flagged	Excellent, with a few style calls

How We Tested Grammarly

Grammarly was tested on a paid Pro account in its web editor, with every tool at its default setting, on one day. The AI detector reads the whole document and shows a single percentage with a "Possible AI text detected" or "No AI generated text detected" line, plus section labels of Strongest, Moderate and Weak AI patterns. We recorded the percentage and the labels for each text.

The 28 texts are the same set we use for every detector: 12 human texts published before 2022 (four academic introductions, three developer docs, two personal blog posts, three essays by non native English learners), 10 plain AI texts from Claude Opus 5.5 and Claude Fable 5.1, and six disguised AI texts written in a hurried student voice or with typos injected by a script. Each runs 280 to 390 words. The GPTZero and ZeroGPT numbers in the comparison come from our October 22, 2026 run, and the Solvely numbers from October 29, 2026.

For the humanizer test we took four plain AI texts, ran them through Grammarly's Humanizer with its "The Naturalist" voice (the one labeled "Replace common AI patterns"), accepted every rewrite, and scanned the result with Grammarly's detector, then with the free checker at zerogpt.com, the Basic Scan at gptzero.me, and Originality.ai 5.0 on a free account. For plagiarism we used a verbatim Wikipedia passage, a verbatim passage from the official Python tutorial, a hand reworded version of that passage, and an unpublished text as a control. For grammar we wrote a 365 word story with 26 planted errors and 10 correct but unusual sentences. Turnitin is not in this test because we have no access to it.

Is the Grammarly AI Detector Accurate? 28 Texts

The Grammarly AI detector is cautious to a fault. It averaged 64.0% on the five plain Opus 5.5 texts and 79.8% on the five plain Fable 5.1 texts, but the individual scores swung from 23% to 99% on texts that GPTZero scored a flat 100%. A plain Opus explainer about Node.js environment variables scored 23% with a single "Weak AI patterns" label. A plain Opus personal finance piece scored 35%.

Disguised AI text got through almost untouched. The three disguised Opus texts averaged 21.7% and the three disguised Fable texts averaged 4.0%. Two of the six scored 0% with the "All clear! Nothing in your text appears to be AI generated" message, and a third scored 12%. Injecting typos into a clean Fable text that had scored 52% dropped it to 0%.

Human writing was where Grammarly looked best. Ten of the 12 human texts scored 0%. The two exceptions were academic introductions at 9% and 15%, each with one "Weak AI patterns" label. Developer docs, personal blog posts and essays by non native English learners all scored 0%, which matters because non native writers are the group most often hurt by false positives.

The full breakdown by text is in the table below. Scores are Grammarly's document level percentage.

Text group	Count	Grammarly scores	Average
Plain Opus 5.5	5	97, 23, 99, 66, 35	64.0%
Plain Fable 5.1	5	99, 99, 89, 60, 52	79.8%
Disguised Opus 5.5	3	48, 0, 17	21.7%
Disguised Fable 5.1	3	0, 12, 0	4.0%
Human academic	4	0, 9, 0, 15	6.0%
Human technical docs	3	0, 0, 0	0%
Human personal blog	2	0, 0	0%
Human non native essays	3	0, 0, 0	0%

Grammarly AI Detector vs GPTZero, ZeroGPT, Solvely and Originality on the Same Texts

Grammarly caught the fewest AI texts of the five detectors we have run on this set. Counting a text as caught when it scores 50% or more, GPTZero caught 16 of 16, Solvely caught 16 of 16, Originality.ai caught 16 of 16, ZeroGPT caught 10 of 16, and Grammarly caught 8 of 16. If you raise the bar to 80%, Grammarly caught 5.

Grammarly's advertising says its detector achieves 99% detection accuracy and ranks first on the RAID benchmark. That may be true on RAID's test set. On our 16 texts from two Claude models released in 2026, it is the weakest of the five at catching AI, and the difference is largest on casual, typo laden writing, which is exactly how a student would hand in AI text.

On human text the picture flips in Grammarly's favor, or at least to a tie. GPTZero gave all 12 human texts a 0% score and a "Human" verdict. Grammarly gave 10 of 12 a 0% score and never went above 15%. Originality topped out at 5%. ZeroGPT gave three human texts scores between 9.5% and 24.9%, and Solvely labeled four human texts "Moderate AI" with a top score of 27.2%. For a writer worried about being accused, GPTZero, Originality and Grammarly are the safest of the five.

Detector	AI texts at 50%+ (of 16)	Highest human score	Human texts at 0%
GPTZero	16	0%	12 of 12
Originality 5.0	16	5%	1 of 12
Solvely	16	27.2%	8 of 12
ZeroGPT	10	24.9%	9 of 12
Grammarly	8	15%	10 of 12

Can Grammarly Humanize AI Text Past Its Own Detector?

Grammarly's Humanizer defeated Grammarly's AI detector on every text we tried. We took four plain AI texts that Grammarly had scored 97%, 99%, 99% and 99%, applied "The Naturalist" voice, and accepted all of the rewrites (three to five per text, one per paragraph). All four rescanned at 0% with the "All clear" message. The rewrites also grew each text by 50 to 80 words and in one case changed "roughly 68 percent" to "around 68%", so the meaning drifted a little as the score fell.

ZeroGPT was fooled too. The same four rewrites scored 0%, 7.7%, 0% and 26.4% at zerogpt.com, an average of 8.5%, down from an average of 99.0% before the rewrite. Three of the four carried ZeroGPT's "Your Text is Human written" label.

GPTZero was not fooled. Its Basic Scan scored all four rewrites 100% AI and attached the beta label "Possible AI Paraphrasing", with the note that it was highly confident the text was originally AI but rewritten by AI or a human. Originality.ai 5.0 was not fooled either: it scored the same four rewrites 88%, 92%, 88% and 88% Likely AI. So a Grammarly Humanizer pass clears Grammarly and ZeroGPT, and walks straight into GPTZero and Originality. If a school or client checks with either of those, the rewrite changes nothing.

Is the Grammarly Plagiarism Checker Accurate?

The Grammarly plagiarism checker is reliable on copy and paste and blind to light rewording. A 578 word verbatim copy of the Wikipedia article on photosynthesis came back as 99% copied text, with five flagged parts and the Wikipedia page named as the source. A 283 word verbatim passage from the official Python tutorial on defining functions came back as 92% copied, with three flagged parts.

For the Python passage, Grammarly named two mirror sites rather than docs.python.org as the matching sources, so the source it shows is not always the original. And when we reworded that same passage by hand, keeping every idea and most of the sentence order but changing the wording, the score fell to 3%, with a single sentence matched to an unrelated documentation page. Our control text, an unpublished 328 word blog post, scored 0% with the message that it "didn't match anything in our billions of academic and online sources".

Test text	Words	Copied text detected	Source named
Wikipedia, Photosynthesis, verbatim	578	99%	The Wikipedia article
Python tutorial, Defining Functions, verbatim	283	92%	Two mirror sites, not docs.python.org
Same Python passage, lightly reworded	333	3%	One unrelated docs page, one sentence
Original unpublished text (control)	328	0%	None

A student who paraphrases a source sentence by sentence will get past Grammarly. A student who pastes it will not.

Is the Grammarly Grammar Checker Accurate? 26 Planted Errors

The Grammarly grammar checker caught 25 of the 26 errors we planted in a 365 word story about a vegetable garden: misspellings like "Niether" and "definately", "alot", "would of", "less gaps", "their/there", "its/it's", "your/you're", "then/than", "to/too", "effected" for "affected", "whom" for "who", a comma splice, a doubled "the the", a lowercase "march", three subject verb agreement slips and two missing commas. The one miss was a tense shift: "The tomatoes come in late, but they were worth the wait" drew no suggestion.

The 10 trap sentences, correct but unusual, were mostly left alone. Grammarly did not touch "the data are", a split infinitive, a sentence starting with "And", "between you and me", "fewer squash plants" or the Oxford comma. It flagged two: it wanted "that" instead of "who" for a family of rabbits, and "were" instead of "was" after "none of them". Both are style preferences, and both come with an Accept button that a trusting student would press.

In total Grammarly made 34 suggestions on the text, 31 under Grammar and 3 under Clarity, and gave it a writing quality score of 57. The seven suggestions that were neither planted errors nor traps were all commas: three before coordinating conjunctions joining full clauses, which most editors would accept, two after short introductory phrases, which are optional, and two rewrites of long sentences. Nothing it suggested would have made the text wrong.

Does Grammarly Give the Same Score Twice?

Grammarly gave the same AI score every time in our repeat runs. We pasted three texts three times each, with the document cleared between runs: the Opus explainer stayed at 23%, the Opus cooking post stayed at 66%, and the human economics introduction stayed at 15%. That is a better result than the complaints on Reddit suggest, where users describe the same story scoring 0% one day and 35% weeks later. Those reports are most likely model updates between sessions rather than noise within a session, and our test cannot rule those out.

Does Using Grammarly Get Flagged as AI?

Accepting Grammarly's spelling and grammar suggestions is editing, and Grammarly's own user guide says that traditional, non generative corrections should not change its AI score. Our test did not measure that case directly, and we have no evidence that fixing a comma moves any detector.

Grammarly's generative tools are a different matter. Every text we ran through the Humanizer came back flagged by GPTZero as originally AI and rewritten, at 100%. If your school or client uses GPTZero, a Grammarly rewrite is visible. If they use Grammarly's own detector or ZeroGPT, it is not. The "Authorship" feature in the editor, which records whether text was typed, pasted or generated, is the only thing in Grammarly that would show the difference, and only if you choose to share that report.

Should You Trust Grammarly?

So, is Grammarly accurate enough to trust? For what it was built for, yes. The grammar checker is the best part of the product and earned its 25 of 26 score. The plagiarism checker is a fine last check against accidental copy and paste. The AI detector is the weak link, and the direction of its weakness matters.

• Students checking their own writing: a 0% from Grammarly is reassuring, because it never flagged a human text in our set. It is not a clearance. A text that passes Grammarly can still score 100% on GPTZero, as all four of our Humanizer rewrites did.
• Teachers and editors screening for AI: do not rely on it. Grammarly missed 8 of 16 AI texts at the 50% line and gave three of them a 0% All clear. GPTZero caught all 16 on the same texts.
• Writers and marketers: use the grammar checker, read its style suggestions with your own judgment, and treat the Humanizer as a rewriting tool rather than a way to pass detection.
• Anyone worried about false accusations: Grammarly, GPTZero and Originality were the three detectors in our set that never pushed a human text past 15%.

On October 31, 2026 Grammarly's plans page listed Pro at $12 a month, and its comparison table put both "Detect AI generated text" and "Catch accidental plagiarism" in the Pro column only. The public AI detector page runs without an account up to 1,400 words, but while signed in it sent us straight into the editor.

FAQ

Is Grammarly 100% accurate?

No. The short answer to "is Grammarly accurate" is that it depends on the tool. In our October 2026 test the Grammarly grammar checker caught 25 of 26 planted errors but also flagged 2 of 10 correct sentences, and the Grammarly AI detector scored only 8 of 16 AI texts at 50% or higher. It was accurate on human text, giving 10 of 12 human texts a 0% AI score.

Can the Grammarly AI detector be wrong?

Yes, mostly in one direction. On our 28 texts it never scored a human text above 15%, but it gave three AI generated texts a 0% score and an All clear message, and it scored a plain Claude Opus 5.5 explainer at 23%. It misses AI text far more often than it accuses human writers.

Is the Grammarly plagiarism checker as good as Turnitin?

We could not test Turnitin, but Grammarly found 99% copied text in a verbatim Wikipedia passage and 92% in a verbatim Python documentation passage. A light rewording of that same documentation dropped the score to 3%, so it catches copy and paste, not paraphrased copying.

Can Grammarly humanize AI text?

Grammarly's Humanizer rewrote four AI texts that scored 97% to 99% on Grammarly's own AI detector, and all four then scored 0%. ZeroGPT scored them 0% to 26.4%. GPTZero still scored all four 100% AI with a Possible AI Paraphrasing label, and Originality.ai scored them 88% to 92%, so the rewrite does not get past every detector.

Is the Grammarly AI detector free?

The public AI detector page accepts up to 1,400 words without an account. Inside the editor, AI detection and plagiarism checking are Pro features, and Grammarly's plans page listed Pro at $12 a month on October 31, 2026.

Does using Grammarly count as AI?

Accepting spelling and grammar fixes is editing, and Grammarly's own guide says those corrections should not change its AI score. Using Grammarly's Humanizer or AI Rewriter is different: GPTZero labeled all four of our Humanizer rewrites as originally AI, rewritten by AI or a human.

Final Thoughts

Grammarly is two products wearing one name. The writing assistant is accurate: 25 of 26 planted errors caught, verbatim copies found at 92% to 99%, the same score on every repeat run. The AI detector is the one people search for and the one that let us down, with 8 of 16 AI texts caught, three AI texts cleared at 0%, and its own Humanizer able to zero every score we tried. If you need to know whether a text is AI, check it with GPTZero or Originality.ai as well. If you need your writing cleaned up, Grammarly still does that better than anything else we have tested.

We run tests like this because we build and deploy language models for clients and need to know what the detectors actually see. If your team is working out how AI generated text fits into its own writing and review process, our custom AI solutions team can help you design the workflow and the checks around it.