Context Window Checker
100% PrivateAI & Prompt ToolsSee how much of each AI model's context window your text fills, model by model.
Runs entirely in your browser. Your input is not uploaded to Ovatly or anywhere else.
Model window sizes come from a data table. Snapshot: November 2026. Prices and limits change; verify with the provider before budgeting. Counts use the GPT tokenizer (o200k encoding, the GPT-4o family). Other model families split text differently, so treat the count as an approximation for them.
- 1Paste the text you plan to send to a model.
- 2Click Check context fill.
- 3Compare the fill bars: each model shows its window size, the percentage your text fills, and the spare tokens.
- 4Pick a model where the text fits with room left for the reply.
The sentence "Prompt engineering is the practice of writing instructions for AI models." is 12 tokens. Against GPT-4o (window 128,000) it fills 0.0094% and leaves 127,988 tokens spare. Against Gemini Flash (window 1,000,000) it fills 0.0012%. The percentages are tiny because real prompts are small next to modern windows; a full book chapter is where the bars start to matter.
- Window sizes come from a snapshot data table dated November 2026; providers change limits, so verify with the provider before relying on a figure.
- Token counts use the GPT tokenizer (o200k encoding), so fills for other families are approximations.
- The check covers your input only. A real conversation also spends tokens on the system prompt, earlier messages and the model's reply.
The bar turns red and the tool states how many tokens you are over. Trim the text (the Prompt Shortener can help), split it into parts, or choose a model with a larger window.
About this tool
The Context Window Checker measures your text in tokens and shows, for each model in its data table, how much of that model's context window the text would fill, how many tokens remain, and whether it fits at all. It is the quick way to see whether a long document, codebase extract or chat transcript can go into a given model before you hit a limit mid-conversation.
Related tools
Part of Ovatly's free tools in AI & Prompt Tools.