Quick answer
AI detectors estimate whether text was machine-written from statistical patterns — predictability of word choice, sentence rhythm, structure. The best commercial tools, GPTZero and Originality.ai, are useful as a signal and unreliable as proof: published research puts most detectors between 50% and 80% accuracy depending on the text, with false positives concentrated on non-native English writers, formal prose, and short passages. Humanised AI text often passes. Use a detector to start a conversation, never to end one.
We build a free AI detector ourselves, so we have a stake in this being said plainly: nobody's detector, including ours, is accurate enough to punish someone on a score alone. What differs between the tools is who they are built for and what evidence they give you beyond a percentage.
GPTZero — built for education
- Sentence-level highlighting shows which passages drive the score, not just an overall number
- Writing Reports replay how a Google Doc was written over time — actual evidence rather than probability
- Plagiarism checking alongside AI detection
- Free for 10,000 words a month; Essential from $10/mo; integrations with Canvas, Moodle, and Google Docs
- Weakness: like all detectors, it flags formal and non-native writing more often than it should
Originality.ai — built for publishers
- Tuned aggressively for the latest models, which catches more AI and flags more humans
- Full-site scans, team seats, and readability scoring for content operations
- Plagiarism and a fact-checking aid in the same tool
- No real free tier: pay-as-you-go from $30 for 3,000 credits, or Pro from $14.95/mo
- Weakness: the aggressive tuning means a higher false-positive rate on genuine human writing
Free tools, including ours
Free detectors — QuillBot's, Grammarly's, and our own — are fine for a quick sanity check and no worse than paid tools at the core task, because the underlying problem is hard for everyone. Ours shows three separate signals with a plain-English explanation of each rather than a single verdict, precisely so you can see how uncertain the estimate is. What free tools lack is the workflow: no plagiarism scans, no team management, no document history.
How to use a detector fairly
- Never act on a score alone; treat a high result as a reason to ask questions, not a finding
- Weight process evidence — drafts, version history, a conversation about the work — above any percentage
- Be especially cautious with non-native writers and technical or formal prose, where false positives cluster
- Remember that humanisers exist and work: a clean score does not prove human authorship either
- Write the policy down before you need it, including what the score does and does not mean
The uncomfortable truth of 2026 is that AI detection is an arms race the detectors are not winning. The tools are worth using for the evidence and workflow they add — not for the verdict.
Related reading
Bottom line
GPTZero for teachers and editors who need evidence, Originality.ai for publishers checking at volume, free tools including ours for a quick read. Whichever you use, the score is the beginning of a conversation.
