01 — Ranking
The 5 best options, ranked
GPTZero
gptzero.me
4.3Popular AI detector built for educators, with sentence-level highlighting.
- Best for
- Teachers reviewing essays and wanting per-sentence explanations.
- Free plan & limits
- The free plan includes a limited number of words per month and per scan. Bulk file scans, plagiarism checks and advanced reports require paid plans (from about $10–15/month).
QuillBot AI Detector
quillbot.com
4.1Free detector from the paraphrasing company, with no sign-up needed.
- Best for
- Quick checks of short texts.
- Free plan & limits
- Free, with a per-scan word limit of about 1,200 words. It distinguishes AI-generated, AI-refined and human text.
ZeroGPT
zerogpt.com
3.6Free detector with a large per-scan character limit.
- Best for
- Scanning longer texts without an account.
- Free plan & limits
- Free with ads, up to roughly 15,000 characters per scan. It has a reputation for false positives on formal or formulaic human writing.
Copyleaks AI Detector
copyleaks.com
4.0Enterprise plagiarism and AI detection with a free web checker.
- Best for
- Checking for both plagiarism and AI content.
- Free plan & limits
- Limited free scans per day on the web. Full plagiarism reports and API access require credits or a subscription.
Originality.ai
originality.ai
4.0Paid detector aimed at publishers and SEO agencies.
- Best for
- Content agencies screening outsourced articles at scale.
- Free plan & limits
- Mostly paid, at about $0.01 per 100 words or a monthly plan. A free Chrome extension offers limited checks. It is stricter than most detectors, so false positives are possible.
02 — Comparison
Side by side
Scroll sideways to see every tool. The first column stays pinned.
| Feature / Criterion | GPTZero | QuillBot | ZeroGPT | Copyleaks | Originality.ai |
|---|---|---|---|---|---|
| Free scans | Monthly word cap | ~1,200 words/scan | ~15,000 chars/scan | Daily cap | Very limited |
| Sentence highlighting | Yes | Yes | Yes | Yes | Yes |
| Plagiarism check | Paid | Separate tool | No | Paid | Paid |
| Account required | For more words | No | No | Yes | Yes |
| Ads | No | Yes | Yes | No | No |
03 — Guide
How to check text with a free AI detector step by step
Use a long enough sample
Detectors are unreliable on short texts. Scan at least 250–300 words of continuous prose. Lists, code, citations and templated text (cover letters, product specs) confuse them.
Run two or three detectors
Scores often disagree by 50 percentage points or more on the same text. If the tools disagree, the result is inconclusive.
Look at highlighted passages, not only the score
Check whether the flagged sentences are generic boilerplate or specific, personal and sourced writing. Highlighted generic passages are weaker evidence than you might think.
Look for process evidence
For students and writers, ask for drafts, version history (Google Docs File → Version history) or a short conversation about the content. These are far more reliable than any detector score.
04 — FAQ
Common questions
How accurate are free AI detectors?
Not accurate enough to make decisions on their own. OpenAI shut down its own AI classifier in 2023 because it correctly identified only about 26% of AI text. Light editing or paraphrasing often fools detectors, while some human writing gets flagged.
Can AI detectors falsely accuse human writers?
Yes. A 2023 Stanford study found that detectors flagged about 61% of non-native English TOEFL essays as AI-generated. Formal, simple or formulaic writing is especially at risk. Several universities have disabled AI detection for this reason.
Does Google penalize AI-generated content?
Google says it rewards helpful content however it is produced, and penalizes content mass-produced mainly to manipulate rankings ('scaled content abuse'). Original insight, accuracy and expertise matter more than whether a tool was involved.
What are the best alternatives if I need advanced features?
Turnitin's AI writing detection is available only through institutions. For publishers, Originality.ai and Copyleaks offer APIs and team dashboards. For academic integrity, many schools now favor oral checks, drafts with version history and assignments designed for AI use over detection.
Scores reflect our own assessment of free-tier value. Prices and limits change, so check them on each provider's site before you buy.