# Will This Fit? — Context Window Calculator

> Paste a document and see which models it fits inside, how much of each window it fills, and what is left for the reply.

This calculator measures a document in tokens and checks it against the context window of every major model, showing how much of each window it fills, whether it still fits once you reserve space for the reply, and what a single call would cost. Message overhead is included, because role markers and separators make a long conversation measurably larger than the text alone.

**URL:** https://convertto.tech/t/context-window-calculator
**Category:** AI & LLM Tools (https://convertto.tech/c/ai-tools)
**Privacy:** Runs entirely in the browser; no upload
**Cost:** Free, no sign-up
**Last updated:** 2026-08-01

## Key facts

- **Largest window here:** 1M tokens — roughly 750,000 words
- **Shared budget:** Input and output come out of the same window on most models
- **Privacy:** Runs entirely in your browser — nothing is uploaded
- **Cost:** Free, unlimited, no sign-up

## How to use

1. Enter or paste your document or prompt.
2. Set the reserve for the reply.
3. Set the messages in the conversation.
4. Choose the count using.
5. Press Run, then download the result when it is ready.

## FAQ

### Does the reply come out of the same window?

Yes on almost every current model — max_tokens is a cap on the reply, but the reply still has to fit alongside the prompt. That is why this tool asks you to reserve space: a prompt that fills 99% of the window leaves no room for an answer.

### How many words is a million tokens?

Roughly 750,000 words of English prose, or about 2,500 pages. Code and JSON are far denser — the same window might hold only 400,000 words of source.

## Sources

- [OpenAI — tokenizer and BPE vocabularies](https://github.com/openai/tiktoken) — OpenAI
- [Anthropic — token counting endpoint](https://platform.claude.com/docs/en/build-with-claude/token-counting) — Anthropic
- [Google — Gemini token counting](https://ai.google.dev/gemini-api/docs/tokens) — Google

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