Token
Also: tokens, token usage
A token is the basic unit Claude uses to read and write text — roughly three-quarters of a word. "Hello, how are you?" is about 6 tokens. Everything you send to Claude (your message, system prompt, uploaded documents) and everything Claude sends back counts as tokens. Tokens determine both your usage limits and your API costs. Understanding tokens helps you manage costs, stay within limits, and structure your prompts efficiently.
In practice
You ask Claude a question that's 50 words. Claude responds with 200 words. Roughly 300 tokens were used in that exchange (a token is about ¾ of a word). Your API bill reflects that usage. Long conversations, big documents, and verbose system prompts all add tokens — and tokens are directly what you pay for.
Related concepts
Where Token shows up
2 articlesTokens are what you pay for. Here are the practical things you can do to use fewer of them — from how you prompt to which model you choose.
Tokens are how language models read and write text — and how every AI API charges you. Understanding them turns abstract pricing into something you can predict and control.