Both names come up constantly in "best AI humanizer" searches, but they're built around different ideas. QuillBot bundles its humanizer into a much bigger writing suite; Undetectable AI is narrowly built around one job — rewriting text to dodge AI detectors — and backs that job with a refund promise. Here's what each one's free tier and pricing actually cover, sourced from their own pricing pages and independent testing, not marketing copy.
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QuillBot is an eight-tool writing suite built around its original paraphraser: Paraphraser, Grammar Checker, AI Detector, Plagiarism Checker, AI Humanizer, AI Chat, Translator, and Summarizer all live in one dashboard. Its humanizer isn't a standalone product — it's built on the same rewriting engine as the paraphraser, which is why both share the same free-tier word cap.
Undetectable AI is narrower by design: two features in one product, an AI-content detector and a separate text-rewriting tool built specifically to make AI-generated text read as human. It doesn't bundle a grammar checker, citation tool, or any of the broader writing features QuillBot ships with — detection and evasion are the entire product.
QuillBot's free plan caps any single paste — including through the humanizer — at 125 words, with only two rewrite modes (Standard and Fluency) available before you hit a paywall, per QuillBot's own pricing page as cited by independent reviewers. Upgrading to Premium removes the word cap and unlocks 9+ rewrite modes, priced at $19.95/month billed monthly or $8.33/month billed annually ($99.95/year), with a 3-day money-back guarantee if you cancel early.
Undetectable AI's free trial is a one-time 250 words total, not a recurring daily or monthly allowance — once it's used, every further rewrite needs a paid plan. Its own pricing page currently lists discounted rates of $5/month (billed annually at $60/year) for 10,000 words/month, up to $21.25/month for 50,000 words/month, with a 6-months-free offer on its top tier; the full list prices shown before that discount run from $9.99 to $42.50/month.
Undetectable AI's pricing page makes a specific, money-backed promise: "If anything we produce is flagged as not human, we will refund the cost of humanization." That's more concrete than most marketing in this space — but it's also only as strong as whichever detector you happen to check against, since detectors routinely disagree with each other. The same company also markets itself as "Rated #1 Best AI Detector by Forbes" for its detection feature specifically, yet a October 2025 retest by journalist David Gewirtz found that detector unreliable in both directions — misjudging genuinely human-written text as likely AI, and AI-written samples as likely human.
QuillBot makes no comparable refund promise. Independent comparisons that test multiple humanizers against detectors like GPTZero and Originality.ai have put its bypass rate at roughly 62% — lower than several purpose-built humanizers in the same comparisons. Figures like this shift constantly as both humanizers and detectors keep updating, so treat any specific percentage — including ones on this page — as a directional snapshot, not a guarantee for your own document.
StudyPilot's AI Humanizer takes a different position than either: it doesn't promise to beat any specific detector or offer a refund tied to one. It gives you 150 words a day free with no account, three rewrite tones (Natural, Simple, Deep rewrite) at that tier, and scores every rewrite before handing it back so you can see where it lands rather than take a guarantee on faith.
Paste your text into StudyPilot's AI Humanizer and pick a tone — 150 words free every day, no account needed, with every rewrite scored before it comes back to you.
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