QuillBot vs Undetectable AI

The free tiers are built on opposite rules. QuillBot's limit is per input and never runs out; Undetectable AI's is a total and never refills. For a reader who wants to see what each tool does to a paragraph, both will show it. For a reader who wants to keep using a free tool, only one will.

QuillBot figures from its Premium page, read 9 November 2026; Undetectable AI trial size checked on 8 November 2026. The arithmetic shows the trade. QuillBot's free tier is a paragraph tool: 125 words is about five sentences, and each input is rewritten without sight of the others, so terms and tense can drift across the joins. Undetectable AI's trial takes a longer single paste but is gone after it. The site's free ranking, the best free AI humanizer list, orders tools by how much text a free limit accepts whole, and both of these sit below the tools with a 1,000-word or larger free input.

The two paid plans are hard to compare because they sell different things. Undetectable AI sells words: 10,000 a month on the entry plan, $5.00 a month billed $60.00 yearly or $9.99 monthly, read 9 November 2026, which is about six 1,500-word essays or twenty 500-word sections. QuillBot sells a suite: Premium at $8.33 a month billed annually lists the paraphraser, grammar checker, plagiarism checker with a 25,000-word monthly allowance, AI detector, humanizer and summarizer, read 9 November 2026, and states no per-input or monthly figure for the humanizer.

A writer who wants only a humanizer and knows the monthly volume has a plain choice in Undetectable AI's plan: the number is printed. A writer who already wants a grammar checker and a paraphraser gets the humanizer thrown in at QuillBot and should read the humanizer's paid cap off the interface, because the Premium page does not print one. Price is reported on this site and never scored.

QuillBot names two modes, Basic on the free plan and Advanced on Premium, with 3 free uses of Advanced, read 9 November 2026, and publishes nothing about what each changes beyond the name. Undetectable AI names no modes on its pricing page, read the same day. For a reader who wants the lightest rewrite that still changes how a text reads, a named ladder is the only control a vendor page offers, and QuillBot's is two rungs tall.

The published method runs each tool on its default setting and on its strongest documented mode where one exists. For QuillBot that means two rows, Basic and Advanced, with the free row on Basic where the 125-word cap accepts the input; the protocol's inputs run 350 to 500 words, so none of them fit the free tier whole. For Undetectable AI the run uses whatever the interface shows and records the name in the raw files.

No peer-reviewed study has tested either product by name in a way this site could cite, so the research here is about paraphrasing as a category. Weber-Wulff and colleagues tested 14 detectors and found every one below 80 percent accuracy, with accuracy falling further on paraphrased and machine-translated text (Weber-Wulff et al. 2023, International Journal for Educational Integrity, read 9 November 2026). Sadasivan and colleagues argued that paraphrasing attacks lower detector accuracy and set out theoretical limits on how reliable detection can be (Sadasivan et al. 2023, arXiv 2303.11156, read 9 November 2026).

Those studies measured detector readings, not whether the rewritten text kept the writer's claims. A rewrite that turns "the study found no effect" into "the study found a small effect" has passed a detector and failed the reader. The method on this site weights meaning kept at 35 points of 100, errors introduced at 20, detector pass rate at 30 and limits and access at 15, and two editors rate every output blind. When a run is published, this page gains meaning, error and per-detector fields for both tools. No detector score should be the only evidence in a misconduct case.