Developer & Tech

Context Window Calculator

What this does

Check whether your system prompt, conversation history and reserved response fit inside a model’s context window.

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Runs in your browser

Calculator inputs

How to use it

Using the context window calculator

  1. 01

    Pick the model’s window

    From its spec sheet; or choose Custom for fine-tuned or preview variants.

  2. 02

    Enter your three budgets

    Count tokens with our counter or your provider’s tokenizer endpoints.

  3. 03

    Mind the verdict row

    Fits confirms acceptance; overflow names exactly how many tokens to shed.

Good to know

One window, three claimants

System prompt, history and completion all compete for the same tokens. Teams that reserve nothing for responses watch replies truncate mid-sentence; teams that hoard reserve pay for unused headroom on every call. Explicit budgets make the trade-off visible.

Long windows aren’t free lunches

Cost scales linearly with context, latency grows, and effective recall degrades toward the middle of very long contexts (“lost in the middle”). Filling 128K tokens because you can is rarely better than curating 16 relevant ones.

How it's calculated

The math behind this calculator

used = system + conversation + reserve utilization = used ÷ window headroom = window − used

Everything sent on the request; system instructions, message history and any space held back for the reply; draws from one shared window. Summing the three budgets against the selected window yields utilization, remaining headroom and a plain fits-or-overflows verdict.

Assumptions & limitations

  • Presets reflect common model offerings (8K–1M); custom covers anything else.
  • Reserving output space prevents truncated replies.
  • Tokenizer variance means keep a safety margin rather than planning to 100%.

Worked example

An 800-token system prompt, 6,000-token history and 2,000-token response reserve use 8,800 of a 128K window; 6.88% utilization with 119,200 tokens spare.

FAQ

Frequently asked questions

Do images count against the window?
Yes; vision models convert images to token equivalents, often several hundred to thousands each depending on resolution.
What happens on overflow?
Providers reject the request outright or silently truncate oldest history; neither is what you want, hence the reserve row.
Is the custom preset needed for new models?
Whenever the spec differs from listed presets; enter its advertised maximum directly.

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