Free Page Speed Budget calculator with clear step-by-step results.
Derives a byte budget from a load-time target rather than picking a round number. Connection setup costs about three round trips, requests queue in parallel batches that each cost a round trip, and rendering consumes real time - whatever is left is what you can spend on bytes.
Overhead
Overhead = 3 x RTT + ceil(requests / 6) x RTT + render time
Byte budget
KB = (target - overhead) x Mbps x 1000 / 8
Because latency eats the budget before any bytes move. On a 120 ms RTT, connection setup and request batching alone can consume over a second regardless of bandwidth.
The target is unreachable at that latency and request count. Reduce requests, use HTTP/2 or HTTP/3 multiplexing, or cut render-blocking work before worrying about file sizes.