GPT-4o Context Window, Max Tokens & 128k Fill %
Last updated: July 2026
What GPT-4o max tokens and 128k mean in practice—fill percentage, reply headroom, and how to check a long paste before the model truncates or forgets.
Queries for gpt-4o max tokens, gpt-4o context window, and 128k tokens ask how much room a paste has inside a published window — not how many characters it has. This guide explains those numbers and how to check before you send.
Max tokens vs context window
“Max tokens” may mean output cap, total context, or both depending on the doc. A 128k figure on a model card usually describes context scale — often shared with the model’s reply. Treating it as 100% available for input alone is a common mistake.
Check fill percentage first
Open the context window calculator, paste text, pick 128k (or 8k / 32k / 200k / 1M / custom). Read used tokens, remaining, and fill %. Trim before truncation or forgotten earlier turns.
Then reconcile the count
For encoding-aware length of the raw string, use OpenAI-style modes on the token counter. The window tool answers “how full”; the counter answers “how long is this.”
What does 128k feel like in words?
For reading intuition only, use the 128k / 1M presets on tokens to words. That is a planning ratio, not a guarantee. Final drafts still need a paste.
Leave room for the answer
If input fills the window, output has nowhere to go. Aim for headroom — not a 100% fill bar.