The formula
needed = users × usage × concurrency × (1 + headroom)
Where it comes from
Adding up every user’s consumption gives a figure that is never reached, because they do not all use the network in the same instant. A concurrency factor is applied — in an office or a home it usually runs around 70 % — and headroom on top of that, because a line running at its limit behaves far worse than one with room to breathe. Ten people watching 4K video at 15 Mbps each do not need 150 Mbps: 126 is comfortable.
How to work it out by hand
- Multiply the users by what each one consumes
- Apply the concurrency factor
- Add the headroom
- Divide by eight if you want the figure in MB/s
What is worth knowing
This sum falls short in practice almost always for the same reason, and it is not the arithmetic: **the upload**. On most home connections the upload is a small fraction of the download, and it is what saturates the moment there are two video calls or a cloud backup running. When a house "feels slow" on a line with plenty of headroom, the culprit is usually a saturated upload, which also drives up latency for everything else. The other common bottleneck is Wi-Fi: a 600 Mbps line is worth nothing if the access point delivers 80 in the back bedroom. Before paying for more speed, measure on a cable next to the router and compare.