GPTZero AI Detector: Review & Free Check
Paste any text below for a free AI score, no account needed. This is Humanit's checker, not GPTZero itself. Under it: how GPTZero scores, what its labels and percentage mean, where it is wrong, and what changed when Grammarly's parent company bought it.
GPTZero estimates whether text is AI-written from perplexity (how predictable the wording is) and burstiness (how much sentence length varies). Its percentage is a probability about the document, not the share of it that was AI-written. Since June 2026 it has been owned by Superhuman, the company behind Grammarly.
Works with every major AI model
How it works
- 1Paste your text. Drop in the writing you want to check against GPTZero-style detection.
- 2Run the free detector. Humanit returns a 0–100 AI-likelihood score, a verdict, and the signals behind it.
- 3See what reads as AI. The subscores show which patterns — predictable phrasing, uniform rhythm, stock vocabulary — are pulling the score up.
- 4Fix it if needed. If it reads as AI, send it to the humanizer to rewrite those signals, then re-check.
What GPTZero measures
GPTZero does not compare your text against a database of AI outputs — no detector can, because models do not keep a record of what they generated. It measures two statistical properties instead. Perplexity asks how surprising each word is given the words before it: language models pick high-probability words, so their output is unusually unsurprising. Burstiness asks how much your sentence lengths vary: humans write a long winding sentence, then a short one, then a medium one, while models tend toward an even rhythm. Low perplexity plus low burstiness is the fingerprint. That is the whole mechanism, and it is why the tool can be fooled and why it can be wrong.
What GPTZero's labels mean
GPTZero no longer answers with just "AI" or "human". Its results now carry labels such as "Human", "AI + Human", "Polished by AI" and "Paraphrased by AI", and some results name a model outright, such as "ChatGPT" or "Claude". Read the model name with the same caution as the score: GPTZero cannot see what any model generated, so pinning text on a particular model is an inference from style, not a lookup. The in-between labels exist because the line between writing with AI help and writing by AI is genuinely blurry — if you drafted something yourself and then ran it through an AI editing tool, "Polished by AI" or "AI + Human" is the kind of verdict that workflow tends to earn, rather than a clean "Human".
What the GPTZero percentage actually means
This is the single most misread number in the category. A "92% AI" result does not mean 92% of your words were machine-written — GPTZero's own documentation describes its headline figure as the probability that the document as a whole is AI-generated, not a proportion of it. Turnitin uses the opposite convention, where the number IS the share of the submission flagged, and so does the Grammarly AI detector, which reports the "% of this text [that] appears to be AI-generated" — worth knowing now that GPTZero and Grammarly share an owner. So a 40% in one tool and a 40% in another can describe completely different things, and comparing scores across detectors is meaningless. If someone shows you a percentage, the first question is which tool produced it.
GPTZero is now owned by Grammarly's parent company
On June 23, 2026, Superhuman — the company Grammarly became after buying the Superhuman email app and taking its name — announced it was acquiring GPTZero, and GPTZero's site now says it "is now a part of Superhuman". The price was not disclosed; GPTZero's co-founder put the startup at 19 million registered users and $30 million in annual recurring revenue. Superhuman's pitch was that "two AI detectors are better than one", and as of September 2026 GPTZero still runs as its own product at gptzero.me, alongside Grammarly's separate detector. Nothing announced so far changes how a GPTZero score is calculated. Free limits and integrations are the kind of thing that shifts after an acquisition, so check GPTZero's own site for current terms rather than trusting any third-party summary, this one included.
Where GPTZero gets it wrong
False positives are documented and they are not rare. GPTZero has been reported flagging the US Constitution as AI-generated — a text written in 1787 — because formal, structured, heavily-edited prose has exactly the low-perplexity profile the model scores on. The most serious finding is about non-native English writers: a 2023 peer-reviewed study published in Patterns tested seven detectors against TOEFL essays and found more than 61% of genuine human essays by non-native speakers were misclassified as AI. The failure mode is systematic, not random — the writing most likely to be wrongly flagged is careful, formulaic, or written by someone composing in a second language.
Why published accuracy figures disagree so wildly
GPTZero's own site advertises near-perfect accuracy and cites a result on RAID, an academic detection benchmark: 95.7% of AI texts caught while 1% of human texts were wrongly flagged. That is a real benchmark figure and it is still a description of one test set. Search for GPTZero's accuracy and you will find other confident figures that contradict each other by a wide margin. They can all be technically correct, because a detector's accuracy is a property of the test set rather than of the tool. Score it on raw, unedited ChatGPT output against clearly human essays and almost any detector looks excellent. Score it on lightly-edited AI text, or on human writing from ESL authors and formal academic prose, and performance drops sharply. The target also moves: every model generation writes closer to human text than the last. Treat any single headline accuracy number — including a vendor's, and including one quoted by a site selling you something — as the result of one experiment on one sample.
What to do if GPTZero flags your writing
If the text is genuinely yours, you are dealing with a false positive, and the practical fixes are the same things that make writing better anyway: vary your sentence lengths deliberately, cut stock transitions like "moreover" and "in conclusion", and add specific, concrete detail that only you would know to include. Keep your drafts, version history, and notes — process evidence is far more persuasive to an instructor than any counter-score. If the draft is AI-assisted and that is permitted for your use, rewrite the flagged signals with a humanizer and verify the result. And if you are the one running the check on someone else's work: a score is a reason to open a conversation, never on its own a reason to conclude one.
Check your text free, before anyone else does
Humanit's detector is above this page. It scores the same family of signals GPTZero uses and returns a 0–100 likelihood broken into sub-signals — perplexity, burstiness, AI vocabulary, discourse markers, structure, personal voice — plus the specific phrases pulling the score up, so you can see why rather than just how much. To be straight about limits: GPTZero's own free scan takes up to 10,000 characters without an account, more than the 300 words ours checks without one. What this checker adds is the breakdown, and a humanizer on the same page if something reads as AI. It is an estimate, exactly like GPTZero's is.
Who it's for
| Feature | GPTZero | Humanit detector |
|---|---|---|
| Free, no account | Up to 10,000 characters per scan | Up to 300 words per check |
| What the score means | Likelihood the document is AI-written | 0–100 likelihood + verdict |
| Result labels | Human, AI + Human, Polished by AI, Paraphrased by AI, model names | Score, verdict and sub-signals |
| Shows why | Highlights the sentences it flags | Sub-signals plus the exact phrases that read as AI |
| Process evidence | Writing Replay | None, it reads the finished text only |
| Fix flagged text in the same place | Detection-focused | Yes, humanizer on the same page |
Frequently asked questions
Is this the official GPTZero detector?
No. This is Humanit's free detector for the same job — paste text, get an AI-likelihood score — so you can check before GPTZero or a teacher does. We do not access your GPTZero account.
Who owns GPTZero?
Superhuman, the company behind Grammarly, which announced the acquisition on June 23, 2026. GPTZero still runs as its own product at gptzero.me.
Is GPTZero the same as Grammarly's AI detector now?
No. They share an owner but, as of September 2026, remain separate tools with different scoring conventions: GPTZero gives a probability for the whole document, while Grammarly reports the share of the text that appears AI-generated.
Is GPTZero accurate?
It is reasonably reliable on raw, unedited AI text and much less so on edited AI text or on human writing that happens to be formal and even-paced. GPTZero advertises near-perfect accuracy; independent results vary widely depending on what was tested, so no single number describes it. Treat a GPTZero result as a signal to look closer, never as proof.
What does an 80% GPTZero score mean?
It is the model's estimated probability that the document is AI-written — not a claim that 80% of your words were generated. Turnitin's and Grammarly's percentages mean the opposite (the share of the text flagged), which is why scores from different detectors cannot be compared.
What does "Polished by AI" mean in GPTZero?
It is one of GPTZero's in-between labels, alongside "AI + Human" and "Paraphrased by AI". It signals text that GPTZero reads as human writing reworked by an AI tool, rather than generated from scratch. Like every label it is an estimate, not a record of what actually happened.
Does GPTZero flag human writing as AI?
Yes, and predictably so. It has been reported flagging the US Constitution, and a 2023 study in Patterns found over 61% of real TOEFL essays by non-native English speakers were misclassified as AI by the detectors tested. Clean, formal, and second-language writing is the most at risk.
Can GPTZero detect ChatGPT, Claude, and Gemini?
It is built to flag output from all major models, and it now sometimes names the model it thinks produced the text, such as "ChatGPT" or "Claude". That attribution is inferred from writing style — GPTZero cannot see what any model actually generated — so treat it as a guess, not a trace.
How do I check my text against GPTZero for free?
GPTZero's site scans up to 10,000 characters without an account; longer scans need a free account. Humanit's detector is free with no account for up to 300 words per check and shows the sub-signals behind its score.
What is the best GPTZero alternative?
It depends what you need. For self-checking before submission, use a detector that shows its reasoning and does not cost anything to run repeatedly — Humanit's is free with no sign-up for up to 300 words per check. For institutional use, the honest answer is that no detector is accurate enough to be used as sole evidence, whichever one you pick.
Which is better, GPTZero or ZeroGPT?
They are different products from different companies, and neither is independently proven better. GPTZero gives a probability for the whole document with labels like "AI + Human" and "Polished by AI"; ZeroGPT highlights sentences and shows an AI percentage, with a larger free check of up to 15,000 characters. Running both and seeing whether they agree tells you more than trusting either one.
Do universities use GPTZero?
Some instructors use GPTZero directly, but institution-wide checking usually runs through a detector plugged into the school's learning platform — most often Turnitin, or Copyleaks, which integrates with Canvas, Moodle, Blackboard and D2L. Ask which tool your course actually uses before reading anything into a GPTZero score.
Is 40% AI detection bad?
It depends entirely on the tool. In GPTZero, 40% is a probability that the whole document is AI-written, which is an uncertain result, not a finding that 40% of your words came from AI. In Turnitin and Grammarly the number is the share of text flagged. No detector publishes a threshold that proves anything, and many schools treat any score as a reason to look closer, not a verdict.
Is there a better AI detector than GPTZero?
No independent test crowns a single best detector, and every vendor's accuracy claim is measured on its own test set. The practical choice is a detector that explains its result and that you can run more than once — and for anything that matters, a second detector as a cross-check.
Will my text be used to train AI?
No. Your text is sent to the model only to produce your result and is never used to train any AI. No content is stored beyond the request.
Does it keep my original meaning?
Yes. Humanit preserves your facts, numbers, names, and citations while changing the phrasing and structure. It never invents new claims.