The AI detection market has exploded since 2023. There are now dozens of tools claiming to identify AI-generated text, but most of them are either partially paywalled, privacy-invasive, or simply not accurate enough to use seriously. This guide cuts through the noise. We tested six of the most commonly used tools across six criteria that actually matter, and we'll tell you which to use depending on your situation.
The Comparison Table
Here's how the six most popular tools compare on the criteria that matter most for everyday users:
| Tool | Free Limit | Accuracy | Privacy | Login | Best For |
|---|---|---|---|---|---|
| AI Detector Free | Unlimited | ~75% | No data stored | Not required | Privacy-focused users |
| GPTZero | 5,000 chars/check | ~80% | Stores text | Required | Educators |
| Copyleaks | 10 credits free | ~78% | Stores text | Required | Enterprise |
| ZeroGPT | Unlimited | ~70% | Unknown policy | Not required | Quick checks |
| Winston AI | 2,000 words free | ~82% | Stores text | Required | Content teams |
| Originality.ai | Paid only | ~85% | Stores text | Required | Content agencies |
Accuracy figures are approximate estimates based on independent benchmark testing. No AI detector achieves perfect accuracy; see the accuracy section below for important caveats.
What "Free" Actually Means
The word "free" means very different things across these tools, and it's worth being precise about the trade-offs. AI detection involves computationally expensive language model inference — running text through neural networks to calculate perplexity and other statistical measures. This costs real money in cloud compute, which means "free" tiers are always subsidised somehow.
Tools like GPTZero and Winston AI use freemium models: the free tier is a loss leader designed to convert users to paid subscriptions. The free limits are set specifically low enough that anyone checking more than a handful of documents per week will need to upgrade. GPTZero's 5,000-character limit, for example, barely covers a typical undergraduate essay.
Our free AI detector runs primarily browser-side analysis using vocabulary and statistical pattern matching that does not require heavy server compute per check. This allows genuinely unlimited free usage without a subscription upsell. The trade-off is that pure browser-side analysis is somewhat less accurate than server-side deep model analysis — which is why our accuracy estimate (~75%) is lower than Winston AI (~82%). For most everyday use cases, 75% accuracy with no limits and no login is a better practical deal than 82% accuracy with a 2,000-word monthly cap.
Accuracy: What the Independent Studies Show
The accuracy figures in the table deserve careful interpretation. No AI detector achieves better than approximately 85–90% accuracy on unmodified AI text under ideal conditions, and real-world accuracy is typically lower. Several important factors affect accuracy:
- False positive rate — Incorrectly flagging human-written text as AI. This is the more dangerous error for use cases like academic integrity, where falsely accusing a student is harmful. Studies show false positive rates of 5–15% for most detectors on native English text, and significantly higher for ESL writers.
- Humanised AI text — Text that has been run through paraphrasing tools like Undetectable AI or QuillBot to evade detection scores dramatically lower on all detectors. Accuracy drops to 40–60% in these cases.
- Short text — Detection accuracy is substantially lower on texts under 100 words. Statistical signals need sufficient sample size to be meaningful. Avoid using any detector as evidence for texts under 200 words.
- Domain effects — Technical writing, medical writing, and legal writing naturally use elevated formal vocabulary that can trigger false positives. Calibrate your thresholds accordingly for these domains.
The practical implication: treat AI detection scores as probabilistic evidence, not verdicts. A score of 85% means the text is more likely AI-generated than not — it does not mean AI authorship is proven. Use detection as one signal among several, alongside manual review for the 7 manual signs of AI writing.
Privacy: The Overlooked Factor
Privacy is the single most underrated criterion in AI detector comparisons, and the one that matters most for certain use cases. When you paste text into an online AI detector, you are sending that text to a third-party server. What happens to it next depends entirely on that company's privacy policy — and most of these policies permit storing and using your text for model improvement.
This matters enormously if the text you're checking is confidential: legal documents, medical records, unreleased creative work, HR communications, client deliverables under NDA, student work under FERPA. Pasting any of this into GPTZero, Copyleaks, or Winston AI may violate your professional obligations or your organisation's data policies.
Our free AI detector processes text client-side in your browser. The text you check is never sent to our servers and we have no ability to store or access it. For anyone handling sensitive documents, this is a meaningful architectural difference, not just a marketing claim. Check our privacy policy for full technical details.
Our Recommendations by Use Case
- Best overall free tool: AI Detector Free — unlimited, no login, no data stored, works for everyday checking
- Best for teachers and educators: GPTZero — has educator-specific features, classroom integrations, and the highest-profile track record in academic settings
- Best for privacy-sensitive content: AI Detector Free — the only major tool that processes text entirely client-side
- Best for bulk checking at scale: Winston AI or Originality.ai (paid) — the free tiers won't cut it for high-volume workflows
- Best for quick spot-checks with no account: AI Detector Free or ZeroGPT — both require no registration
The bottom line: if you're a casual user or anyone handling sensitive content, our free AI detector is the right starting point. If you're an educator needing classroom-specific features, GPTZero's free tier is worth the login friction. For high-volume professional use, budget for a paid tool — the free tiers of premium products aren't designed for that workflow.