What Is an AI Text Detector Trying to Do?
An AI text detector looks at a piece of writing and guesses if a person or an AI tool wrote it. It cannot see how the text was made. It can only look at the words on the page and compare them with patterns it knows.
That is why the answer is always a guess, often shown as a percentage. A high number means the text has many AI-like patterns. It does not mean the text was definitely written by AI.
Perplexity: How Predictable Is the Text?
Perplexity is a way to measure how surprised a language model is by the next word. Take the sentence "The cat sat on the ...". Almost any model expects the word "mat". That is low perplexity. If the next word is "submarine", the model is surprised. That is high perplexity.
AI tools write by picking likely words, so their text often has low perplexity. People are less predictable. We use odd words, jokes, and half-finished thoughts. So some detectors treat very predictable text as a sign of AI.
The catch is that some human writing is also very predictable, such as legal text, simple instructions, or writing by someone who is still learning English.
Burstiness: How Much Does Sentence Length Change?
Burstiness is about rhythm. People often write a short sentence. Then a longer one that adds detail and explains the idea. Then short again. AI text tends to keep sentences closer to the same length, so the rhythm feels flat.
Detectors measure this by counting the words in each sentence and checking how much the numbers vary. Little variation gives a higher AI score. The term became popular through detectors such as GPTZero.
Other Signals: Word Lists and Trained Models
Some detectors also look for words that AI tools use too often, like "delve", "tapestry", and "furthermore". Read more in our guide to overused AI clichés.
Bigger detectors use a trained classifier. Its makers show it many human texts and many AI texts, and it learns to tell them apart. That can work better on the kind of text it learned from, but it can fail on new AI models, edited text, or writing styles it has not seen.
What Our Detector Does
Our free AI text detector is simple on purpose. It does not measure perplexity. It checks 25 common AI words and phrases, sentence length variation, sentences of the same length, and passive voice. It shows the reason for every flagged sentence, so you can judge it yourself. Everything runs in your browser and your text is not uploaded.
Why No Detector Is Ever Certain
AI models change fast, people edit AI drafts, and many people write in a plain style. All of this blurs the line. In 2023, OpenAI closed its own AI text classifier because of its low accuracy. So use any score as one clue, and check it against what you know about the writer and the draft. Our next guide explains why detectors flag human writing.