A bit is the basic unit of information in computing and telecommunications; it is the amount of information that can be stored by a device and can normally exist in only two distinct states. A bit can also be defined as a variable or computed quantity that can have only two possible values. These two values are often interpreted as binary digits and are usually denoted by digits 0 and 1. ISO/IEC IEC 80000-13:2008 specifies that the symbol for bit should be "bit", though the letter "b" (in lower case) is still widely used.

byte (B) also octet(o)

A byte or octet is a unit of memory or data equal to the amount used to represent one character; The size of a byte is typically hardware dependent, but the modern de facto standard is 8 bits. However, byte has been used for numbers of bits other than 8. To avoid the risk of confusion ISO/IEC IEC 80000-13:2008 strongly recommends that the name byte and the symbol "B" (capital letter) be used only for 8-bit bytes. A computer with 8-bit bytes can distinguish 2^{8} = 256 different characters. The symbol "B" is not international and should not be confused with the symbol "B" for bel

Prefixes for binary multiples
Computers use bits (binary digit), which are composed of ones and zeros. The decimal number system in contrast has ten unique digits, zero through nine. But although computer data and file size is normally measured using the binary number system (counted by factors of two 1, 2, 4, 8, 16, 32, 64, etc), the prefixes for the multiples were based on the metric/SI system because computer industry noticed that 2^{10} was very nearly equal to 1000 and started using the SI prefix "kilo" to mean 1024! So, although a metric/SI "kilo" equals 1,000 (e.g. one kilogram = 1,000 grams), a binary "Kilo" equals 1,024 (e.g. one Kilobyte = 1,024 bytes). Not surprisingly leading to confusion, particularly when many storage device manufacturers and telecommunications engineers use the decimal prefixes. So a "500 GB hard drive" may show only 465 GB (500,000,000,000 bytes) on the computer. No Surprise, some may feel they have been cheated out of 35 GB. A big potential for confusion and for incompatibility in standards and in implemented systems exists.

In December 1998, the International Electrotechnical Commission (IEC) approved a new standard. Instead of using the metric prefixes for multiples in binary code, the new IEC standard called for specific prefixes for binary multiples made up of only the first two letters of the metric prefixes and adding the first two letters of the word "binary". Instead of Kilobyte (KB) or Gigabyte (GB), the new terms would be kibibyte (KiB) or gibibyte (GiB).

Summary

The common use of metric/SI prefixes for binary multiples can lead to confusion over whether value is binary or decimal. Use of "k" for decimal kilo and "K" for binary helps but not with other prefixes like mega (M) and giga (G). Symbol b for bit and B for byte is used to differentiate, but is still subject to confusion.

New IEC standard should make things clearer. Confusion may still exist with use of old and new metric/SI prefixes.

KiB - kibibyte - 1024 (2^{10}) bytes
MB - megabyte - 1000000 (10^{6}) bytes. (SI prefix for decimal use only - may cause confusion with old system).

Symbol

Prefix

SI

Binary

Size difference

k

kilo

10^{3} = 1000^{1}

2^{10} = 1024^{1}

2.40%

M

mega

10^{6} = 1000^{2}

2^{20} = 1024^{2}

4.86%

G

giga

10^{9} = 1000^{3}

2^{30} = 1024^{3}

7.37%

T

tera

10^{12} = 1000^{4}

2^{40} = 1024^{4}

9.95%

P

peta

10^{15} = 1000^{5}

2^{50} = 1024^{5}

12.59%

E

exa

10^{18} = 1000^{6}

2^{60} = 1024^{6}

15.29%

Z

zetta

10^{21} = 1000^{7}

2^{70} = 1024^{7}

18.67%

Y

yotta

10^{24} = 1000^{8}

2^{80} = 1024^{8}

20.89%

Symbol

Prefix

Ki

kibi, binary kilo

kibibyte (KiB)

2^{10} bytes

1024 B

Mi

mebi, binary mega

mebibyte (MiB)

2^{20} bytes

1024 KiB

Gi

gibi, binary giga

gibibyte (GiB)

2^{30} bytes

1024 MiB

Ti

tebi, binary tera

tebibyte (TiB)

2^{40} bytes

1024 GiB

Pi

pebi, binary peta

pebibyte (PiB)

2^{50} bytes

1024 TiB

Ei

exbi, binary exa

exbibyte (EiB)

2^{60} bytes

1024 PiB

Zi

zebi, binary zetta

zebibyte (ZiB)

2^{70} bytes

1024 EiB

Yi

yobi, binary yotta

yobibyte (EiB)

2^{80} bytes

1024 ZiB

Data Transfer Rate - bits per second and bytes per second are confusing to those outside the specific industries. Terms such as 56K, 56 kb/s, 56Kb/s, 56 kbps, 56 Kbps, 56 kBps, 56 KBps etc. don't help. In most cases bitrate prefixes are used in the decimal sense.