Megabits and megabytes are different
Mbps means megabits per second, while MB/s means megabytes per second. Because one byte contains 8 bits, divide Mbps by 8 to get theoretical MB/s.
A 100 Mbps connection therefore has a theoretical maximum of 12.5 MB/s. Lowercase b means bits and uppercase B means bytes.
Estimate download time
Convert file size and transfer rate to compatible units before dividing size by speed. A 1,000 MB file at 12.5 MB/s takes 80 seconds in theory.
You can instead multiply megabytes by 8 to get megabits, then divide by the Mbps rate. Both methods produce the same result.
Why real downloads are slower
Advertised speed is usually a maximum link rate. Wi-Fi, congestion, server limits, protocol overhead, device performance, and other traffic reduce throughput.
For planning, include a margin rather than assuming the full advertised speed will be sustained.
Decimal and binary rate labels
Network rates typically use decimal prefixes, so 1 Mbps means 1,000,000 bits per second. Some software reports rates using MiB/s.
Check whether an application displays MB/s or MiB/s before comparing it with an internet plan. Similar labels produce different large-file estimates.