Estimate token count and context window usage before making API calls.
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What is a token? Language models don't read text character by character — they break it into "tokens," which are chunks of roughly 3–4 characters for common English words. For example, "calculator" might be 2–3 tokens, while a single letter like "a" is 1 token.
Estimation method: This calculator uses the widely-cited rule of thumb of approximately 4 characters per token for typical English text. This gives a reasonable estimate for planning purposes. Non-English text, code, and special characters may tokenize differently (code is often more tokens per character; some languages like Chinese may be fewer characters per token).
Why this matters: API pricing is based on token count. Context windows have hard limits — if your input exceeds the limit, the API will reject the request or truncate the text. Knowing your usage upfront helps you design prompts that fit within budget and limits.
For exact counts: Use the official tokenizer tools — OpenAI's Tokenizer playground, Anthropic's token counting API endpoint, or Google's Vertex AI token counters — before production use.