Convert bits to bytes. Divide by 8, and see why a 100 Mbps connection downloads at 12.5 MB/s.

The formula

bytes = bits ÷ 8

A byte is eight bits. This has been effectively universal since the 1960s, though it is a convention rather than a law of nature — some early machines used six-bit or nine-bit bytes, and the standards bodies use octet when they need to be unambiguous.

The lower-case b is the whole problem

Symbol	Means
`b`	bit
`B`	byte
`Mbps` or `Mb/s`	megabits per second
`MB/s`	megabytes per second

One character of difference, a factor of eight in meaning. Broadband is sold in bits; file sizes and download progress bars are in bytes. That mismatch is why a connection that sounds fast appears to download slowly.

Broadband speeds

Sold as	Megabits/s	Megabytes/s	1 GB file takes
50 Mbps	50	6.25	2 min 40 s
100 Mbps	100	12.5	1 min 20 s
300 Mbps	300	37.5	27 s
500 Mbps	500	62.5	16 s
1 Gbps	1000	125	8 s
2 Gbps	2000	250	4 s

Real throughput is a few percent below these figures — TCP, IP and Ethernet headers all take a share, and the last stretch of the path is rarely the bottleneck-free case. But the factor of 8 accounts for almost all of the gap between the advertised number and what the download shows.

Quick reference

Bits	Bytes
8 b	1 B
1000 b	125 B
1 kbit	125 B
1 Mbit	125 kB
1 Gbit	125 MB

Where bits are the natural unit

Bits are not merely a smaller byte. They are the right unit wherever the quantity is not a whole number of bytes:

Network throughput, because serial links move bits and the framing is not byte-aligned in any useful sense.
Colour depth — “24-bit colour” is 8 bits each for red, green and blue.
Audio and video bitrates — a 320 kbps MP3 uses 40 kB per second.
Key lengths — a 256-bit key is 32 bytes.
Addressing — a 64-bit pointer is 8 bytes.

Estimating in your head

Divide by 8, or halve it three times. For broadband, the shortcut is to divide by 10 and add a quarter: 300 Mbps ÷ 10 is 30, plus 7.5 is 37.5 MB/s, which is exact.

Bits to bytes