Megabits vs Megabytes: Speed in Bits, Storage in Bytes

What is the difference between megabits and megabytes?

A megabit (Mb) is 1 000 000 bits and a megabyte (MB) is 1 000 000 bytes, which is 8 000 000 bits.

The prefix is the same in both words. Mega means a million here, exactly as it does in megawatt, so the whole gap between the two units comes from what is being counted: single bits in one, bytes in the other. A byte is 8 bits, and that factor of eight carries straight through to the megabyte.

Both units measure an amount of data, so one always converts into the other with the same fixed factor, whatever the data is. A megabit is not a slower megabyte or a different kind of file; it is an eighth of the size. You can convert a size between bits, bytes and megabytes and read the factor in the working.

To check a figure by hand, count a number of bits as bytes and read the division by eight in the working. Going the other way, turn a byte count back into bits, which multiplies by eight.

What do lowercase b and uppercase B mean?

A lowercase b is the bit and an uppercase B is the byte, so Mb and Mbps count megabits while MB and MB/s count megabytes.

The convention is written down in JEDEC's dictionary of terms, which lists the bit as b and the byte as B. The M in front means the same in both, so the case of the one letter after it is the only thing that changes the size, and it changes it by a factor of eight.

NIST's table of prefixes is stricter still. It writes the bit out in full, as in kbit, and keeps B for the byte alone, so a rate written as 100 Mbit/s cannot be misread as bytes. When a label is typed all in capitals or all in lower case, the letters stop telling you which unit is meant, and the context has to: a rate from a network link is counted in bits, and a file on a disk is counted in bytes.

Why are internet speeds in megabits but file sizes in megabytes?

Each number is written in the unit its own specification counts in. A link is described by how many bits cross it each second, and a file is described by how many bytes it holds.

A data transfer rate uses decimal prefixes: 1 Mb/s is 1 000 000 bits per second.

JEDEC's dictionary gives that rate as its example of the decimal prefix, and NIST notes that telecommunications engineers use the megabit per second to mean a million bits per second. US broadband rules keep the same habit, and put the two units side by side:

47 CFR 54.308 states broadband speed in Mbps and data allowance in GB: at least 10 Mbps downstream, with a minimum usage allowance of 150 GB per month.

A file's size is counted in bytes: POSIX defines st_size, the size of a regular file, in bytes.

So the two numbers answer different questions. The plan speed says how fast data can move; the file size says how much of it has to move. Before you divide one by the other to estimate a download, both have to be in the same unit first.

How do you convert Mbps to MB/s?

Divide by 8. A rate in megabits per second becomes a rate in megabytes per second in the same way a count of bits becomes a count of bytes, because the "per second" is shared and only the unit in front of it changes.

To turn Mbps into MB/s, divide by 8; to turn MB/s into Mbps, multiply by 8.

1. Take the rate in megabits per second: 100 Mbps.
2. Divide by 8, the number of bits in a byte: 100 / 8 = 12.5.
3. Write the result in megabytes per second: 12.5 MB/s.
4. Check it the other way round: 12.5 x 8 = 100, which is the rate you started with.

100 Mbps is 12.5 MB/s before protocol overhead.

The divided figure is a ceiling for the link, not a promise for the file, because part of every packet is header rather than file:

Every IPv4 packet carries a header of at least 20 bytes and every TCP segment another 20, and those bytes use the link's rate without being part of the file.

That is why a real download reads somewhat under the divided figure. The unit a program counts in moves the number as well.

12.5 MB/s is the same rate as 11.92 MiB/s. The two figures differ because a megabyte and a mebibyte are not the same size.