How much text can an AI detector scan? The real limits.
Most free AI checkers cap a paste somewhere between a few hundred words and 15,000 characters. ScanForAI scans 20,000 characters free with no account — 50,000 with a free one — so a full paper gets judged in one pass instead of in chopped-up chunks.
Paste limits across the field.
Checked against each tool's own pages on August 2, 2026. Where a limit isn't clearly documented, the table says so — we'd rather write "unverified" than invent a number.
| Free / no account | With an account or paid | Notes | |
|---|---|---|---|
| ScanForAI | 20,000 chars (~3,500 words), no account | 50,000 chars free account · 100,000 planned for Pro | Over the limit? Extra is grayed and skipped, never rejected. Pricing |
| GPTZero | Free plan metered in words per month (10,000/mo per 2026 reviews); per-paste cap not clearly documented — reviews report 5,000–10,000 chars | Paid plans raise the monthly word allowance | Per-scan paste limit: varies / unverified — check gptzero.me |
| ZeroGPT | 15,000 chars per detection (their own input counter) | Paid plans advertise more (numbers not verified) | Verified on zerogpt.com, Aug 2026 |
| QuillBot | 1,200 words (~7,000 chars) per scan | Premium: "no word limit for detection scans" | Verified on quillbot.com, Aug 2026 |
| Originality.ai | No open free scanner — limited trial scans; 100-word minimum | Credit-based: 1 credit per 100 words; AI scans max 10,000 words each | Verified in their help center, Aug 2026 |
| Copyleaks | Free checker allowance is small; per-paste cap varies / unverified | Credit-based: 1 credit per 250-word "page"; API accepts up to 500,000 words per document | API figure from docs.copyleaks.com, Aug 2026 |
Competitor details paraphrased from public pages and help centers, last reviewed August 2, 2026, and may change — check gptzero.me, zerogpt.com, quillbot.com, originality.ai and copyleaks.com for current terms. All trademarks belong to their owners; ScanForAI is not affiliated with or endorsed by any of them. We publish no accuracy percentages for anyone — here's why.
A small limit doesn't just cost clicks — it distorts the score.
Detection signals are document-level: sentence rhythm across pages, how vocabulary spreads, whether the ending sounds like the beginning. Squeeze a 6,000-word thesis chapter through a 5,000-character window and you're not scanning the document — you're scanning fragments of it.
Chunks disagree with each other
Split one document into four pastes and you get four scores judged on partial evidence. Which one do you act on? A single pass gives one result you can actually discuss.
Short text is where detectors fail
Every detector — ours included — is least reliable on short input. Chopping to fit a limit converts one long, well-evidenced scan into several short, shaky ones. We say "too short to judge" instead of guessing; fragments invite guesses.
Whole papers, one pass
20,000 characters covers a typical essay or article with no account; 50,000 (free account) covers a dissertation chapter or a long report. See the real enforced limits.
Boundaries cut sentences
Chunk borders land mid-paragraph, and the signals that read flow across sections are blind at the seams. The distortion isn't hypothetical — it's built into any workflow that pastes a document in pieces.
What happens when you paste more.
Nothing breaks. On the free scanner, text past 20,000 characters is grayed out in place and skipped — the scan runs on what fits, tells you the extra wasn't scanned, and never throws your paste away. A free account raises the limit to 50,000 characters and unlocks the full breakdown. And as always: anonymous scans are never stored.
Common questions.
Which AI detector can scan 20,000 characters for free?
ScanForAI scans 20,000 characters (about 3,500 words) with no account, and 50,000 characters with a free account. Among the tools whose limits we could verify in August 2026, that is the largest clearly documented free paste limit — but limits change, so check each tool's own pages before relying on a number.
How many words is 20,000 characters?
Roughly 3,500 words of English prose — a full essay, a long blog post, or a typical term paper in one scan. 50,000 characters is roughly 8,500 words; 100,000 characters is roughly 17,000 words.
What happens if I paste more than the character limit?
On ScanForAI the scan still runs: the text past the limit is grayed out in place and skipped — it is never rejected and you never lose your paste. A free account raises the limit from 20,000 to 50,000 characters.
Why do character limits matter for AI detection?
Detection signals read document-level patterns — rhythm, vocabulary spread, how sections relate. Chop a long document into chunks to fit a small limit and each chunk gets its own score on partial evidence, the scores disagree, and short chunks are exactly where every detector is least reliable. One pass over the whole document gives one consistent, discussable result.