Are AI Detectors Accurate? We Tested 6 Free Detectors On The Same Text

Short answer: free AI detectors are good at catching obvious, unedited AI text, and in our test none of them flagged a classic human-written passage. But when we gave them an AI draft that had been rewritten to sound natural, they split down the middle. Two said it was AI (correct), and two said it was human.

So a detector score is a signal, not a verdict. Below is exactly how we tested, every score with screenshots, and how to use these tools without being misled.

How We Tested

On 26 October 2026 we ran three texts of roughly 180 to 195 words each through the free versions of six popular AI detectors, using the same browser session and no paid plans:

1. Raw AI draft (194 words). A generic blog intro about remote work, generated by AI with no edits. Full of stock phrases like “In today’s fast-paced digital landscape” and “In conclusion”.
2. Human text (182 words). The opening paragraphs of Jerome K. Jerome’s novel Three Men in a Boat, published in 1889. Long before AI existed, so we know a human wrote it.
3. AI draft, rewritten (192 words). The same remote work draft, rewritten by AI to sound human: a first-person story, specific numbers, short sentences, contractions and an opinion. It is still AI text, so the correct answer here is “AI”.

The detectors: GPTZero, ZeroGPT, QuillBot, Sapling, Copyleaks and Grammarly. We recorded the overall “AI” percentage each tool gave. Where a tool’s free tier stopped us before we could test a sample, we say so instead of guessing.

AI Detector Results At A Glance

Detector	Raw AI draft	Human text (1889)	AI draft, rewritten
GPTZero	100% AI	0% AI	Not scored (free scans ran out)
ZeroGPT	100% AI	0% AI	13.1% AI
QuillBot	100% AI	0% AI	0% AI
Sapling	100% AI	0.3% AI	93.9% AI
Copyleaks	100% AI	0% AI	100% AI
Grammarly	100% AI	Not scored (free scan used up)	Not scored

Every detector caught the raw AI draft. Every detector that scored the human text got it right. The rewritten AI draft is where accuracy fell apart: Sapling and Copyleaks caught it, QuillBot cleared it completely, and ZeroGPT gave it a low 13.1%.

Detector-By-Detector Results

1. GPTZero

GPTZero was confident and correct on both samples it scored: 100% AI for the raw draft and 100% human for the 1889 passage. It also highlights which sentences look most AI-like, which is useful when you need to edit.

The catch is the free limit. After two scans in our session, GPTZero asked us to create an account, so we could not score the rewritten draft without signing up.

2. ZeroGPT

ZeroGPT scored the raw draft 100% AI and the human passage 0%, with sentence highlighting on each. It did not stop us with a sign-up wall.

Its result on the rewritten draft is the most confusing in the whole test. The score was 13.1% AI, but the headline above it read “Your Text is Most Likely AI/GPT generated”. The number and the label point in opposite directions, which shows why you should never rely on a single headline.

3. QuillBot

QuillBot got the first two right (100% and 0%) and gives the most helpful explanation of any tool: click a highlighted sentence and it tells you why it was flagged, such as “overly formal tone”, and lists the phrases it matched.

It was also the easiest detector to fool. The rewritten AI draft scored 0% AI and 100% human. The free plan shows a daily scan counter, so heavy users will hit a cap. Note that Scribbr’s free AI detector displays QuillBot branding in its results panel, so we did not count it as a separate tool.

4. Sapling

Sapling was the most accurate tool we could test on all three samples: 100% for the raw draft, 0.3% for the human text and 93.9% for the rewritten AI draft. It needs no account, scores in seconds and gives a shareable report link.

Sapling’s own page warns that no detector, including its own, should be used as a standalone check because false positives and negatives happen. That is good advice for every tool on this list.

5. Copyleaks

Copyleaks matched Sapling: 100% AI for the raw draft, 0% for the human text and 100% for the rewritten draft. It also counts AI words against human words and flags “AI phrases” (28 in the raw draft, 2 in the rewrite).

The one downside is that it gives a hard 100% on text that a person might have heavily edited. If you edit AI drafts by hand, Copyleaks may still call the result fully AI.

6. Grammarly

Grammarly’s free detector scored the raw AI draft 100% AI. After that one scan, it stopped returning results and asked us to sign up, so we could not score the other two samples on the free version.

What The Results Tell You

Obvious AI text is easy to catch

All six tools scored the untouched AI draft at 100%. If someone pastes ChatGPT output without editing it, any of these detectors will notice.

Edited AI text is a coin flip

Once the AI draft was rewritten with specifics and a natural voice, two of four detectors called it human. Which tool you choose decides the verdict. We cover the editing side in How To Make AI Text Sound Human (We Tested It On 4 AI Detectors).

Labels and numbers can disagree

ZeroGPT’s 13.1% with a “Most Likely AI” headline shows that the summary line on a detector is not always consistent with its own score. Read both, and look at which sentences were highlighted.

One clean human result does not prove a low false-positive rate

Our human sample is a famous public-domain novel, and that is only one passage. False positives are a real problem in practice. Stanford researchers reported in 2023 that several detectors flagged essays by non-native English writers as AI far more often than essays by native speakers. Plain, formal or formulaic human writing can trip detectors too.

Which AI Detector Should You Use?

Detector	Correct calls in our test	Free limit we hit	Best for
Sapling	3 of 3	None	Quick checks without an account
Copyleaks	3 of 3	None	Strict checks, AI phrase counts
GPTZero	2 of 2 scored	Sign-up after 2 scans	Sentence-level highlighting
ZeroGPT	2 of 3 (score), label inconsistent	None	Fast, no-login screening
QuillBot	2 of 3	Daily scan cap	Explaining why text reads as AI
Grammarly	1 of 1 scored	Sign-up after 1 scan	Users already on Grammarly

Our practical advice:

• Use at least two detectors. In our test, one tool alone could have cleared AI text or flagged it, depending on which you picked.
• Read the highlights, not just the score. The flagged sentences tell you what to rewrite, which is more useful than a percentage.
• Never use a score as proof. For teachers, editors and clients, a high score is a reason to ask questions, not a final answer. Several detector vendors say the same on their own pages.
• Focus on quality. Google says it judges content on how helpful it is, not on how it was made. See Does Google Penalize AI-Generated Texts?

Looking for related tools? See our reviews of the top plagiarism checkers, the best AI writing tools in 2026 and our Grammarly vs QuillBot comparison.

Limitations Of This Test

• Three samples is a small test. It shows how these tools behave, not a precise accuracy rate.
• We used free versions only. Paid plans may use different models or allow longer texts.
• Detectors update their models often. Scores for the same text can change from month to month.
• Free-tier limits stopped us from scoring three of the eighteen combinations.

Conclusion

Are AI detectors accurate? For raw, unedited AI text, yes: all six caught it. For human text, the five tools that scored our passage got it right. For AI text that has been rewritten to sound human, accuracy dropped to two out of four. Treat any single score as a starting point, compare at least two tools, and judge the writing itself.

FAQ

Which free AI detector is the most accurate?

In our test, Sapling and Copyleaks were the only tools that scored all three samples correctly, including the rewritten AI draft.

Can AI detectors be wrong about human writing?

Yes. None flagged our 1889 passage, but published research has found detectors misjudge some human writing, especially by non-native English writers.

Can edited AI text pass AI detectors?

Sometimes. Our rewritten AI draft passed QuillBot (0%) and scored 13.1% on ZeroGPT, but Sapling (93.9%) and Copyleaks (100%) still caught it.