Download time calculator

The arithmetic is size in bytes times eight, divided by speed in bits per second. The factor of eight is the whole difficulty: broadband is sold in megabits per second and files are measured in megabytes, so a 100 Mbps line moves 12.5 MB every second at absolute best. A 5 GB game takes about seven minutes on that line, not fifty seconds.

The theoretical figure is never reached. TCP/IP headers, acknowledgements, retransmissions and TLS all consume capacity, and the shared segment between you and your exchange is contended with your neighbours. Fifteen per cent is a fair default for a wired connection; over wifi, twenty-five to forty is more honest, because the airtime is shared and half-duplex.

Upload is the same sum with a different number in it, and that number is usually much smaller. Cable and ADSL are asymmetric by design: a "500 Mbps" package commonly uploads at 20 or 50, and the advertised figure is always the download one. Full fibre is often symmetric, which is the practical reason it matters to anyone who sends large files. The asymmetry is also why a video call degrades the moment someone else starts a backup. The upload path saturates first, and that is the direction your own camera needs.

The speed-needed mode runs the sum backwards: a volume of data and a window give 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, because queuing delay rises sharply as utilisation approaches capacity, so keep sustained use below about 70% if latency matters at all. For concurrent users the number that counts is the simultaneous peak, not the headcount; thirty people at 3 Mbps is 90 Mbps in theory and less in practice, because they are not all talking at once.

The two conversion modes exist because the units are distinguished only by letter case. Lowercase b is a bit, uppercase B is a byte, and it is the only unit in everyday use where capitalisation changes the value by 800%. Network units are also strictly decimal: a megabit is 1,000,000 bits, never 1,048,576, so converting Mbps to MB/s is a clean divide by eight with no binary correction anywhere in it. The binary prefixes belong to storage, where a "1 GB" file in a Windows listing is really 1 GiB: 1,073,741,824 bytes, seven per cent more than a decimal gigabyte, and enough to notice on a large transfer.

One thing worth checking before blaming the connection: the source. A transfer runs at the slowest link in the chain, and a server limiting you to 5 MB/s will do so whether your line is 100 Mbps or gigabit. If several simultaneous downloads each run at the same speed, the limit is at the far end.