Grammarly vs QuillBot, Tested: What the Free Plans Really Do, and Whether Their AI Detectors Get You Wrong

Quick answer: on the free plans, QuillBot gives you more. It fixes grammar well, rewrites a clunky email for free, and its AI detector got every sample right. Grammarly's free plan is excellent at catching mistakes once you make an account, and its free AI detector also got every sample right, but you only get about 3 scans a day, and rewriting and the in-app detector are Pro.

What we tested

We gave both tools the same four tests, first with no account (what a first-time visitor sees), then with free accounts (what most people use). Every test text is printed below so anyone can repeat it.

The big question: will a detector say your own writing is AI?

We ran five kinds of text through the detectors: the Declaration of Independence, the Gettysburg Address, a paragraph written by AI, an unpublished paragraph our founder CW dictated, and that same paragraph after a grammar cleanup and after an AI rewrite.

Result: both detectors scored every human text, including CW's raw words, as human, and both correctly flagged the AI paragraph. QuillBot also scored CW's cleaned-up and AI-rewritten versions as human. That matters, because polishing your writing is not the same as AI writing it, and a tool that treats them as the same is making a false accusation. Grammarly's daily free limit ran out before we could check the polished versions there. We'll add that result.

Small test, honest limits: a handful of samples is not a lab study, and famous texts like the Gettysburg Address may simply be recognized. A good or bad result here doesn't prove a detector is always right or always wrong.

How AI detectors actually decide

Detectors don't know who wrote something, and they don't measure how smart it is. They mostly score two patterns: how predictable the word choices are, and how even the sentences are. AI tends to pick the most expected words in tidy, similar-length sentences. So careful, smooth, very correct human writing can sometimes look "AI-like" too.

That's also why results can differ from writer to writer. In a 2023 Stanford-led study, several detectors flagged English essays by non-native English writers as AI far more often than essays by U.S. students. The likely reason is the method, not anyone's intent: writers working in a second language often use simpler, more common words, which is exactly the pattern detectors score. Tools read best the kind of writing they saw most. OpenAI shut down its own AI-text detector in 2023 for low accuracy.

Our advice: treat a detector score as a hint, never a verdict. If you're a teacher or a boss, always have a person take a second look before accusing anyone. QuillBot says the same thing right on its results: "Never rely on AI detection alone to make decisions that could impact someone's career or academic standing." That's the honest move.

Claude's verdict

If you're choosing a free tool, start with QuillBot. It does more for free, and it's upfront about what its detector can't do. If you mainly want mistakes caught as you type, Grammarly's free account is excellent at that. Just know that most of the rewriting and AI-check features it shows you are Pro.

Things to know: QuillBot's paraphraser page says "No Sign-up Required," but it asked us to sign up on first use without an account. Its free rewrite and paraphrase limits are short (about 80 to 100 words). Grammarly's free AI detector allowed about 3 scans a day signed in.

Test texts (so you can repeat it)

Planted errors: "Their going to the libary tomorow to finish there group project, but nobody have the notes from last week. Me and Jordan was suppose to bring them, however we forgot. The professor said the deadline is firm, its due friday at noon. If we dont finish on time, we loose ten percent of are grade."

Rambling email: "hi, so i just wanted to reach out and touch base about the thing we talked about last week because i think it would be really good if we could maybe possibly get it done sooner rather than later if thats ok with you and everyone else on the team, let me know thanks"

Paraphrase facts: "The city council voted 5 to 2 on Tuesday to raise the minimum wage to $15 an hour starting January 1, 2027. The two members who voted no said small businesses need more time to adjust."

Sources

• QuillBot and Grammarly, free plans, tested Oct 18, 2026
• Liang et al. (2023), Patterns: detector results on non-native English writing
• The Decoder: OpenAI discontinues its AI text detector

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