GPTZero vs QuillBot AI Detector: AI detector comparison
A side by side look at pricing, methodology, language support and best-fit use case. Every claim on this page is linked to a source at the bottom.
Last updated . Reviewed by IsItAI Editorial.
At a glance
GPTZero
US, 2023The original AI detector, popular with educators in the US.
- Pricing
- From $14.99/month (Essential)
- Free tier
- 10k words/month free
- API
- Yes
- Languages
- 7
QuillBot AI Detector
US, 2017 (AI detector 2023)Free detector from the company best known for paraphrasing tools.
- Pricing
- Free (Premium suite from $9.95/month)
- Free tier
- Free unlimited
- API
- No
- Languages
- 3
Side by side
| Feature | GPTZero | QuillBot AI Detector |
|---|---|---|
| Free tier | 10k words/month free | Free unlimited |
| Starting price | From $14.99/month (Essential) | Free (Premium suite from $9.95/month) |
| Cost per 1k words | $0.02 on Pro plan | Free |
| API available | Yes | No |
| Languages supported | 7 | 3 |
| Founded | 2023 | 2017 (AI detector 2023) |
| Country | US | US |
| Primary audience | US schools and universities | Students and casual writers |
| Best for | US educators familiar with the brand and its 'perplexity and burstiness' explanation | Quick free checks alongside QuillBot's paraphraser |
| Known weakness | Documented false-positive issues on non-native English writing in peer-reviewed studies | Conflict of interest. The same company sells the paraphraser that defeats detectors |
How each tool works
GPTZero
Method: Perplexity and burstiness analysis on a sentence-by-sentence basis. Founded by Edward Tian.
Data handling: US hosted. Stores submissions for model improvement unless on enterprise plan.
QuillBot AI Detector
Method: Free classifier within the QuillBot writing suite.
Data handling: Stores submissions for product improvement.
How accurate are AI detectors? What the research says
Two findings from independent academic research apply to almost every detector on this list, including GPTZero and QuillBot AI Detector.
1. False positives on non-native English writers. Liang et al. (Stanford, 2023) found that GPT detectors flagged over half of TOEFL essays written by non-native English speakers as AI-generated, while flagging almost none of the essays from US students. If your use case involves non-native English writers, never treat a detector verdict as proof on its own.
2. Paraphrasing reliably defeats classifiers. Sadasivan et al. (Maryland, 2023) showed that running AI output through a paraphraser drops detection performance close to chance. Both GPTZero and QuillBot AI Detector are vulnerable to this.
For a deeper breakdown of how accuracy claims hold up across 2026 testing, see our guide to how accurate AI detectors actually are in 2026.
Where GPTZero wins
- Published, explainable method (sentence-level perplexity and burstiness) rather than a free classifier bundled into a paraphrasing suite.
- API access for teams building detection into a workflow. QuillBot's detector has no API.
- Wider language coverage at 7 languages versus 3, with a track record in US education.
Where QuillBot AI Detector wins
- Completely free and unlimited, where GPTZero caps its free tier at 10,000 words per month.
- No signup needed for a fast one-off check.
- Sits next to QuillBot's paraphraser and grammar tools that students already use.
Which one should you pick, GPTZero or QuillBot AI Detector?
Pick GPTZero if you want a published method, API access, or broader language support and you are making a decision that has consequences. Pick QuillBot only for a free, casual check, keeping in mind a clear conflict of interest: the same company sells the paraphraser built to defeat AI detectors. GPTZero has documented false positives on non-native English writing (Liang et al. Stanford 2023), and paraphrasing can defeat either tool (Sadasivan et al. 2023), so treat both as a screening signal, not proof.
Frequently asked questions
Is GPTZero free?
10k words/month free
Is QuillBot AI Detector free?
Free unlimited
Can either tool be fooled by paraphrasing?
Yes. Sadasivan et al. (2023) demonstrated that paraphrasing reliably defeats most AI classifiers. Treat any detector verdict as a screening signal, not proof.
Are AI detectors fair to non-native English writers?
Independent research (Liang et al. 2023) found high false-positive rates for non-native English writing. Always combine a detector signal with other evidence, never use it as a sole basis for an academic or hiring decision.
Which one should I pick if I have to choose one, GPTZero or QuillBot AI Detector?
Pick GPTZero if you want a published method, API access, or broader language support and you are making a decision that has consequences. Pick QuillBot only for a free, casual check, keeping in mind a clear conflict of interest: the same company sells the paraphraser built to defeat AI detectors. GPTZero has documented false positives on non-native English writing (Liang et al. Stanford 2023), and paraphrasing can defeat either tool (Sadasivan et al. 2023), so treat both as a screening signal, not proof.
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Sources and further reading
- GPTZero homepage
- GPTZero pricing
- Liang et al. (Stanford, 2023): GPT detectors are biased against non-native English writers
- QuillBot AI detector
- Sadasivan et al. (2023): Can AI-generated text be reliably detected?