Free tool
Context Window Calculator
Paste your prompt and instantly see how much of each model's context window it uses — GPT-4o, Claude, Gemini and more — including room reserved for the response.
GPT-4o
3.1% used
GPT-4o mini
3.1% used
Claude (Sonnet / Opus / Haiku)
2.0% used
Gemini Pro
0.4% used
Gemini Flash
0.4% used
Llama 3.1
3.1% used
Mistral Large
3.1% used
DeepSeek
3.1% used
Context windows shown are common defaults and change between model versions — treat this as a planning estimate.
Frequently asked questions
What is a context window?
The context window is the maximum amount of text — measured in tokens — a model can consider at once, including your prompt, any documents, the conversation history and its own response. Exceed it and the earliest content is dropped.
Why reserve tokens for the response?
The context window covers input and output together. If you fill it entirely with your prompt, the model has no room left to answer. Reserving 2,000-4,000 tokens is a sensible default.