Free AI detectors: what you give up, and what you do not
Free usually means one of four business models, and which one you are using determines what happens to the text you paste in.
There are a lot of free AI detectors and they are not free for the same reasons. Working out which model you are looking at tells you more about a tool than its accuracy claim does.
We give ours away with no account and no limit. The four models below are still the four models.
The four ways a free detector pays for itself
Free tier, paid product. The free check is capped, usually a few hundred words, and the real product is a subscription. Perfectly reasonable, but the cap matters: short text is where every detector is least reliable, so the free tier is the version most likely to hand you a number that carries the least weight.
Free detector, paid rewriter. Common among the humanising tools. The detector exists to produce an alarming number and the rewriter sells the cure. Worth knowing when reading a score from one.
Advertising. The check is genuinely free and the page is monetised around it. Fine, though it creates a quiet incentive toward dramatic results, because a page that says "no problem found" gets less engagement.
Free because it costs almost nothing to run. Ours. The heavy part runs in your browser rather than on our servers, so an extra check costs us close to zero and there is nothing to meter. That is the entire reason we can leave it uncapped.
What actually matters more than the price
Whether it publishes a false positive rate. Accuracy blends missing generated text with flagging text somebody wrote, and only one of those lands on a person. Most free tools publish neither. Turnitin, which is not free, publishes under 1% of documents and around 4% of sentences. Ours is 0.8% at the public setting and 0.4% stricter, on pre-LLM academic writing.
Whether it scores per passage. A single document percentage hides where the signal is. A 30% spread evenly means something quite different from a 30% in one section, and the number alone cannot separate them.
The word cap. A cap that forces you to check a paper in fragments is not a small inconvenience, it degrades the result, because every detector is far more reliable per document than per sentence.
What happens to your text. Worth checking before pasting unpublished work into anything. Some tools store submissions and some say nothing at all about it.
Why a free score and a paid score disagree
Not because one of them is better. Detectors disagree for structural reasons: different training data, different thresholds, different segmentation, different weighting of the underlying signals. Two tools can compute the same internal score and report different verdicts purely because of where each drew its line. See why AI detectors disagree.
Wide disagreement is informative rather than embarrassing. It usually means the text sits near the boundary, which is worth knowing before anyone acts on either number.
Using a free check well
- Run the whole document, not an extract, if the tool lets you
- Prefer per-passage output over a document percentage
- Run two tools and treat the disagreement as the finding
- Discount short text, non-native writing and heavily templated sections
- Ask for the false positive rate, and note when there is not one
Ours is free and unlimited with no account, takes .pdf, .docx and .tex, and scores each paragraph separately. The method is in how the unslop AI detector works.