Web Performance Budget Calculator

A performance budget is a commitment that the page will not exceed a certain weight. It works because it inverts the question: instead of measuring the time once everything is built and trying to patch it, you set the ceiling first and every new resource has to fit. When somebody wants to add a 180 KB library, the conversation stops being about taste and starts being about what comes out to make room.

Enter the target time and the connection and the page works out how many KB fit. Fill in the current weight by resource type and it compares against the budget, estimates the load time and sorts the resources heaviest first — which is where cutting starts. The connection profiles are the ones used by auditing tools, and there is an option to enter your own throughput and latency.

The part usually forgotten is latency. Before the first useful byte arrives, DNS has to resolve, the connection has to open, TLS has to negotiate and the response has to come back — four round trips in the figure used here. On slow 3G, at 400 ms per round trip, that consumes 1.6 seconds before any content. Ignoring that share inflates the budget by 25% or more, which is why the page subtracts it before dividing.