Download time

What it's for

Handy to know how long you'll wait to download a movie, a game, a backup or a work file, and to compare different internet plans.

Mbps or MB/s?

Watch the unit: connections are measured in megabits per second (Mbps, lowercase «b»), not megabytes. 1 byte = 8 bits, so 100 Mbps is about 12.5 MB/s of real download speed.

Why the real time is nearly always longer

The sum here is the ideal one: size divided by speed. In reality something is lost, and knowing where helps you tell a problem from normality. Part of it goes on the protocol (headers, checks, retransmissions): that's 5-10% and it doesn't come back. The rest nearly always depends on a bottleneck that isn't your line.

The three commonest, in order: the WiFi, which on a gigabit line is almost always the slowest link in the chain, especially at 2.4 GHz or through two walls; the server at the other end, which if it's far away or busy gives what it can; and the computer's disk, which on a traditional spinning drive struggles to write more than 100-150 MB a second. If a download is slow, before blaming the provider it's worth trying with a cable.

Upload speed is a different matter

Nearly all home connections are asymmetric: the speed you download at is much higher than the one you upload at. On a mixed fibre-copper line the upload can be a tenth of the download, and full fibre often reaches a third. It's the number that counts when you send a video, run an online backup or are on a video call while somebody else is uploading: for those cases the time has to be worked out with the upload speed, not the advertised one.

Nearby tools

To see how much a file really weighs there is Byte converter, and for a video's size before exporting it Video bitrate and size. If the file is too big to send, the pages are Compress video, Compress PDF and Split a PDF under a size.