Download time calculator
A 100 Mbps connection is 100 million bits per second, which is 12.5 million bytes — 12.5 MB/s. A 5 GB file therefore takes at least 400 seconds, not 50. Connections are sold in bits and files are measured in bytes, and that single factor of eight is behind almost every "my internet is slower than advertised" complaint.
Works out how long a download takes from the file size and your connection speed, converting between bits and bytes correctly and letting you allow for protocol overhead.
How to use the 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.
One more thing worth checking before blaming the connection: the source. A download runs at the slowest link in the chain, and a server limiting you to 5 MB/s will do so regardless of 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, not yours.
The binary units are offered because they still matter in the places that use them honestly. A "1 GB" file quoted by a Windows file listing is really 1 GiB — 1,073,741,824 bytes, seven per cent more than a decimal gigabyte — which is enough to notice on a large transfer.
Questions
Almost always the factor of eight: connections are sold in megabits and files are measured in megabytes. A 100 Mbps line downloads at about 12.5 MB/s at best.