One megabit is 0.125 megabytes, because there are eight bits in a byte. This single factor of eight is the reason a 100 Mbit connection never downloads at 100 MB per second, and it accounts for more confusion than any other unit relationship in consumer technology.
About the units
A megabit is a million bits; a megabyte is a million bytes, and therefore eight million bits. Network engineering measures throughput in bits per second because that is what a serial link physically transmits — one bit at a time down a wire or a fibre. Storage measures in bytes because that is the smallest addressable unit of memory. Both conventions are correct in their own domain, and the collision happens in the user interface, where the connection is advertised in megabits and the download manager counts megabytes.
The exact factor
The factor is exactly 0.125, or one eighth. A 100 Mbit/s connection has a theoretical ceiling of 12.5 MB/s; a gigabit connection tops out at 125 MB/s. Real throughput is always lower, because framing, protocol headers and acknowledgements consume part of the link — typically five to ten per cent for TCP over Ethernet, more on wireless. So a 100 Mbit line delivering a steady 11 MB/s is performing normally, not underperforming.
Where you meet this conversion
Broadband advertising and the complaints it generates. Video streaming bitrates are quoted in megabits per second — roughly 5 Mbit/s for 1080p and 25 Mbit/s for 4K — while the resulting files are measured in megabytes, so estimating how much data an evening of streaming consumes requires the conversion. Backup windows, file transfer estimates and video conferencing requirements all cross the same boundary.