Free AI Detector Tools Compared: Limits and Formats

Disclosure: ProofAI is our product. This page is published by 4Born Solutions, which makes ProofAI. To keep the comparison fair, every competitor fact below comes from that tool's own official page, which we opened on 16-17 October 2026. All competitor limits are shown as seen on 2026-10-17. Where a page did not state something, we say "not stated" rather than guess. We do not rank tools by accuracy, because we have not run an independent test and did not find one we could cite for all of these tools.

Free tiers change often. Competitor limits below are as seen on 2026-10-17; check each link before you rely on a limit.

Try it: use ProofAI's AI detector tool, free for 10 checks a day with no login.

Comparison table

Tool	Free limit (as stated)	Input formats (as stated)	Account needed for free use?	Source
ProofAI	10 checks a day without an account; 25 a day with a free account. Text minimum 50 words	Pasted text; PDF, DOCX, DOC, TXT; JPG, PNG, WebP images; files up to 25 MB	No	proofai.in
GPTZero	Up to 10,000 characters per scan without an account; longer scans need a free account. Monthly limit not stated	Pasted text; document upload mentioned, formats not stated	No, for scans up to 10,000 characters	gptzero.me, checked 17 Oct 2026
ZeroGPT	Text box shows a 15,000-character limit. Checks per day not stated	Pasted text; file upload option, formats not stated	Not stated	zerogpt.com, checked 17 Oct 2026
Originality.ai	Up to 2,000 words and 3 free scans a day. A 100-word minimum is stated for the platform generally	Paste, upload or URL; file formats not stated	No	originality.ai, checked 17 Oct 2026
Copyleaks	Up to 25,000 characters per scan, per Copyleaks. Scan count not stated	Upload or paste; file formats not stated	Not stated	copyleaks.com, checked 17 Oct 2026
Sapling	Free check covers the first 2,000 characters	Pasted text; PDF and DOCX with selectable text	Not stated	sapling.ai, checked 17 Oct 2026

A few details from the official pages that may matter to you:

• Originality.ai says text entered into its free AI detector "may be used for training". If you are checking confidential documents, read each tool's privacy policy first.
• Sapling says scanned images need OCR before upload, since the text must be selectable.
• ProofAI shows the passages behind a score in its main detector, offers a REST API, and sells paid plans in INR. It does not detect AI video yet. ProofAI sends the text or files you check to AI model providers to analyse them and keeps a record of each check (date, type and score, never your content); see our privacy policy.

Tools we removed from the earlier version

The earlier version of this post also covered Writer.com, Content at Scale and Crossplag. As of 17 October 2026, Writer's AI content detector URL redirects to writer.com's homepage, which does not mention a detector. The Content at Scale detector URL redirects to brandwell.ai and then to a moxby.com marketplace page. Crossplag's detector URL redirects to inspera.com (Inspera acquired Crossplag in 2023), and the Inspera page we were sent to returned a 404 error. Because we could not confirm any of these tools' current free offer from an official page, we left them out.

Also read: How to Screen AI-Written Job Applications Fairly

Why we do not rank these tools by accuracy

Vendors' own accuracy claims are hard to compare, and published research shows why care is needed.

• OpenAI's own AI text classifier correctly flagged 26% of AI-written text in its test, mislabelled human text 9% of the time, and was withdrawn on 20 July 2023 "due to its low rate of accuracy" (OpenAI).
• A Stanford study by Liang et al. tested seven detectors on 91 TOEFL essays written by Chinese students and found an average false-positive rate of 61.3% (Times Higher Education).
• Vanderbilt University disabled Turnitin's AI detector in 2023, noting that even Turnitin's claimed 1% false-positive rate could have meant about 750 wrongly flagged papers out of the 75,000 it submitted in 2022 (Vanderbilt University).

These studies covered specific tools at specific times, and detectors have changed since. The lesson that still applies is that any detector, including ours, can be wrong in both directions.

How to choose a free AI detector

Pick based on what you actually need to check.

1. Length. Stated free limits vary widely, from the first 2,000 characters (Sapling) to 25,000 characters (Copyleaks) per scan. A long essay may not fit some free tiers in one go.
2. File type. If you mostly receive PDFs or Word files, choose a tool that states it accepts them, so you avoid copy-paste errors.
3. Privacy. Read the privacy terms, especially for student work, resumes or client documents.
4. Explanations. A tool that highlights which passages drove the score lets you review the reasoning instead of trusting a single number.
5. Volume. If you check many documents, look at daily limits and whether an API or paid plan is available in your currency.

How to use any detector responsibly

• Treat results as probabilities. A high score is a reason to look closer, not proof.
• Test longer samples. OpenAI found its classifier was "very unreliable" on texts below 1,000 characters.
• Read the flagged passages yourself. Check whether facts and sources hold up.
• Be careful with second-language writers. The Liang study shows they face a higher risk of false flags.
• Talk to the author before acting. Drafts, notes and version history are better evidence than any score.

Sources

• ProofAI free AI detector. /free-ai-detector
• GPTZero homepage, checked 17 Oct 2026. https://gptzero.me/
• ZeroGPT homepage, checked 17 Oct 2026. https://www.zerogpt.com/
• Originality.ai homepage, checked 17 Oct 2026. https://originality.ai/
• Copyleaks AI content detector, checked 17 Oct 2026. https://copyleaks.com/ai-content-detector
• Sapling AI content detector, checked 17 Oct 2026. https://sapling.ai/ai-content-detector
• Writer AI content detector URL (redirects to the writer.com homepage), checked 17 Oct 2026. https://writer.com/ai-content-detector/
• OpenAI. New AI classifier for indicating AI-written text (updated July 2023). https://openai.com/index/new-ai-classifier-for-indicating-ai-written-text/
• Times Higher Education (11 July 2023). AI text detectors 'biased against non-native English speakers'. https://www.timeshighereducation.com/node/727071
• Vanderbilt University (16 Aug 2023). Guidance on AI detection and why we're disabling Turnitin's AI detector. https://www.vanderbilt.edu/brightspace/2023/08/16/guidance-on-ai-detection-and-why-were-disabling-turnitins-ai-detector/

Written and reviewed by the 4Born Content Writer Team. AI tools assisted with drafting; every statistic above links to its source.