Grammarly AI Humanizer Review: I Tested It Against Three Detectors

On 25 September 2026, I fed a 129-word paragraph of AI filler to Grammarly's free AI humanizer and handed the output to two AI detectors. Grammarly's own detector said 0% AI. Sapling said 99.1% AI. Both were looking at the same 134 words.

The spread is the whole review. Grammarly's AI humanizer is a good copy editor. It is not a way past a detector and the one number on Grammarly's own site that suggests otherwise comes from a tool Grammarly also sells.

Here is what I did, what came back and who this is for.

What you get without an account

The humanizer lives at grammarly.com/ai-humanizer. Paste in the text to be rewritten on the left and then press Humanize. Read the rewritten text on the right. The word counter at the bottom of the box reads 1,500 words.

You get one rewrite. I tested this twice, once on 20 September and once on 25 September and both times pressing Humanize for the second time opened a modal window that read "Get Grammarly for free to get more" with a Create an Account button. However, the output panel only showed the first rewrite, making it seem as though the tool had been run again. Watch the text, not the button.

Behind the account are the features advertised for the page: four different voice styles, a custom voice that can be trained on a writing sample provided by the user and humanizing in six different languages (English, Spanish, French, German, Portuguese and Italian). On the Grammarly plans page as of 25 September was the Free plan for €0 a month and the Pro plan for €12 a month, with the Enterprise plan being sold through Superhuman Go. Note which line is in the Pro column: "Detect plagiarism and AI generated text." The detector that said my text was 0% AI is the free sample of a feature that costs money.

The test

For each tool review I publish on my blog, I use three different text samples. Sample A is a paragraph of text that is typical of essays written by AI systems, containing 129 words and discussing the topic of social media. Sample C is a paragraph of human-written text from William Strunk Jr.'s The Elements of Style, which was published in 1918 and is out of copyright.

Sample A was fed into the free humanizer without any changes to the output. The output was taken exactly as received from the humanizer and fed into Sapling's AI detector. Sample C went through as a control, because a detector that declares everything to be AI is of little use. I also ran the same sample through our own humanizer at RewriteAI and scored that output the same way, since a review of a competitor that quietly skips the comparison is not worth reading.

One thing worth noting before the scores are given: the humanizer did not lose a single fact. Facebook, Instagram and X all came through intact. Plenty of humanizers will quietly drop a date or a name while rephrasing the sentence but this one did not.

What the detectors said

Sapling's scores are from 25 September 2026, Grammarly's from 20 September.

Read the control row first. Sapling puts 98.8 points between the human paragraph and the AI one, so it is not returning a constant. When it calls a humanized passage AI, that is a judgement rather than a default.

Now read the two middle rows. Grammarly's rewrite moved Sapling's score by nine tenths of one percentage point. Ours moved it by four tenths. Neither of those is a detector doing anything but shrugging. If the goal is to change what an AI detector says about your document, neither tool achieved it, and I would rather print our own number next to theirs than leave it out.

Grammarly's own detector is the odd one out

On 20 September, I ran three separate AI-written passages through the free detector at grammarly.com/ai-detector while logged out: sample A, Grammarly's own rewrite of sample A and a fresh paragraph I wrote in the most characteristic machine register I could manage, the kind with a "first and foremost" and a "thoughtful and measured approach" in it. Each of these samples returned 0% AI, with the gauge reading "No AI text patterns found: 100%".

The same page claims that the detector achieves 99% detection accuracy and ranks #1 on RAID's independent benchmark.

Two honest caveats. I did not create an account, so I do not know how the detector performs within a paid Grammarly session. The free page is also rationed: on 25 September it gave me one scan before asking me to sign up, where five days earlier it had allowed three. However, the free version of the tool did not identify text created by AI in instances where other tools did.

These findings are generalizable beyond Grammarly. When the same company produces both a humanizer and a detector, the verdict of the detector on the output of the humanizer is not evidence of anything but marketing. A humanizer and a detector built by the same company have every reason to agree with each other, and none of those reasons is evidence. That is why every score in the table above comes from a company with no interest in the outcome. The problem of detector accuracy is discussed in more detail in our article on what the published accuracy figures actually say, as well as in our review of Grammarly's detector.

What it actually changed

The rewrite is a genuinely good edit. Compare the two opening sentences:

Social media has fundamentally transformed the way in which individuals communicate with one another in the modern era.

Social media has completely changed the way people communicate with each other in the modern day.

Nine words shorter, three registers plainer. "Platforms such as Facebook, Instagram, and X have created unprecedented opportunities for people to connect across vast geographical distances" became "Sites like Facebook, Instagram, and X have offered people unprecedented chances to stay in contact over long distances". The tool strips the vocabulary that makes writing sound like a press release. It is doing what Grammarly has always done well.

What it does not touch is the structure of the text. The sentences were left in place and in the same order as they originally appeared in the text. This is the structure that the current detectors are built to read. The classifier is looking at the predictability of each word in relation to the words that came before it. Replacing words like "fundamentally transformed" with words like "completely changed" will not change the structure of the text enough for the current detectors to fail to detect it. This is the same failure of the humanized text that I discussed in my earlier post on why humanized text is still detected.

For those who want to test this tool themselves, another thing to note is that I fed the same paragraph to the tool on both the 20th and the 25th and the last sentence came out differently on both occasions ("political debate, educational methods and personal relationships" vs "political discussion, methods of education and personal relationships"). Since there are no seeds or version numbers indicated on the website, any results you receive from this tool will be the results of a single run of the tool, not of the tool itself.

Where it fits

If you have drafted something with AI, you are allowed to use AI for it and you want it to stop sounding like a chatbot, this is a good free option for one paragraph at a time. It is also a good option for writers who are working in a second language and want to make sure that the writing is plain but still contains the facts. Grammarly's own page states that you should follow whatever rules apply to you regarding disclosing the use of AI in your writing.

If your goal is to produce a submission that a detector will not flag, then this is not the tool for you. Neither is ours for text like sample A. That paragraph, rewritten by RewriteAI's humanizer, still scored 99.6% at Sapling. A paragraph that is generic in its bones will always be generic, regardless of who rewrites it. What moves a score is specific content carried in sentences that vary, which is a writing problem before it is a tooling one. The value of a humanizer is in working with text that already has something to say; ours is built for that purpose, not for turning generic text into something else.

The short version

Grammarly's AI humanizer is free for one rewrite of up to 1,500 words without an account. The tool is a good plain-English editor that preserved every fact in my test. It is not a detector workaround: it moved Sapling's score on my sample from 100.0% to 99.1%, and our own humanizer only reached 99.6% on the same paragraph, which is the same shrug. Grammarly's own free detector returned 0% on three different AI passages, including the humanized one, which is a reason to distrust same-vendor scoring rather than a reason to trust the humanizer. The output is not deterministic either, so a score you get is a score for one run. Use it to tidy up writing you are permitted to submit, and expect nothing from it at the detector.

If you want the longer treatment:

• How AI detectors work — what the classifiers measure, and why word swaps do not move them
• Are AI detectors accurate? — every published accuracy figure in one table
• Why humanized text still gets detected — the structural reasons, with fixes
• Grammarly AI detector review — the detector half of the same product
• RewriteAI's humanizer and what a Grammarly flag actually means — the two pages behind the comparison above