Does a prompt that barely fits always work well?

Not necessarily. Leaving headroom for output, tool messages, system instructions, and application-added context is safer.

● Runs locally in your browser

AI Context Window Checker

Q: What is a context window?

It is the token capacity available to a model for the request context and, depending on the model, generated output.

Q: Is the token estimate exact?

No. This static checker uses a cross-model estimate, so provider-side token counts can differ.

Paste a prompt, document, transcript, or code sample and see which leading AI model context windows can fit the planned request.

Paste the content you want to fit

Expected output tokens

Estimated input0

Total planned tokens2,000

Private: text remains in your browser.Plan headroom: system/tool context may add tokens.Pricing data verified —

Model fit

Which context windows can hold it?

The status includes your estimated input plus the expected output amount above.

Model	Context	Max output	Status	Estimated remaining

Application frameworks can prepend system messages, tool definitions, memory, retrieved documents, or other hidden context. Leave operational headroom instead of targeting 100% utilization.

Planning

A context window is a budget, not a target.

Even when the raw text fits, real applications often add instructions, tool schemas, retrieval context, and conversational history.

For production workloads, measure provider-reported usage and reserve enough space for the output you actually need.

Input + outputThe checker considers both when evaluating a request.

Output capsA model may have a context window larger than its maximum generation limit.

Estimate cost nextTake the token result to the API Cost Calculator.

What 100K tokens is~75,000 words, 150 pages, and the chunk size that stays under OpenAI's 272K surcharge: How Many Words Is 100K Tokens?.

Context windows explainedInput and output share one budget — window sizes, output caps and what it costs to fill one: What Is a Context Window?.

What a token isThe unit behind every estimate — ~4 characters, 0.75 words: What Is an AI Token?

Sending scraped HTML?Measure what cleaning it saves: HTML Token Savings.

1M tokens in words750,000 words, 1,500 pages — and why only 5 of 8 models accept it in one request: How Many Words Is 1M Tokens?

Trim the promptFind repeated instructions, markup and filler: Prompt Weight Analyzer

FAQ

Context-window questions

What is a context window?

It is the token capacity a model can work with for a request, subject to the provider's specific input and output rules.

Should I use the full context limit?

Usually not. Leave headroom for system instructions, tools, retrieval, conversation history, and output.

Is the token estimate exact?

No. The current static version estimates across model families rather than running each provider's exact tokenizer.