The megabit, unit of digital data

The megabit is one million bits, written Mb. It is the unit of speed rather than of size: a connection is sold in megabits per second, while the file it carries is measured in megabytes.

This is the one place in the catalogue where a single lowercase letter separates two units served side by side. Mb is the megabit, MB is the megabyte, and the eight bits of a byte set them apart by a factor of eight. The habit is not an accident: a network moves bits one after another along a wire, so it counts what it actually transports, while a disk stores characters and counts bytes. The two conventions meet on the invoice, where 100 Mb/s becomes 12.5 MB/s in the download window, and the disappointment is old enough to have its own name in support scripts.

The megabit in other units

Digital storage