Bandwidth calculator

This is the question behind most capacity planning: a nightly backup of 50 GB with a six-hour window, a video feed that must not stall, an office of thirty people. Working back from data and time gives the floor, and the floor is not what you buy.

The second figure adds twenty per cent, and that is the one to provision against. A link running at its theoretical capacity has no headroom for retransmissions or bursts, and queuing delay rises sharply as utilisation approaches 100% — the standard rule of thumb is to keep sustained utilisation below about 70% if latency matters at all. For a backup window where only throughput matters, running closer to the limit is fine.

For concurrent users the arithmetic multiplies, but not linearly in practice. Thirty people on video calls at 3 Mbps each is 90 Mbps in theory; contention ratios mean the real peak is lower, because they are not all talking at once. The number that matters for an office is the simultaneous peak, not the headcount.