Bit, Byte, Word, Kilo, Mega — What do They Mean?

By Jan Plaza

Units of memory capacity or amount of information

b bit  
B Byte 1Byte = 8 bits
  word Depends on computer architecture.
1 word = 32 bits in older desktop computers and laptops.
1 word = 64 bits in present desktop computers and laptops.

While bytes (MB, GB, TB) are used to describe memory capacity and file sizes,
bits are used while discussing bandwidth, the speed of transmitting information (Mb/sec).

Prefixes

k kilo 1000  103 1000
K binary kilo 1024 210 1024
m mega 1000*1000 106 1,000,000
M binary mega 1024*1024 220 1,048,576
g giga 1000*1000*1000 109 1,000,000,000
G binary giga 1024*1024*1024 230 1,073,741,824
t tera 1000*1000*1000*1000 1012 1,000,000,000,000
T binary tera 1024*1024*1024*1024 240 1,099,511,627,776

1KB is approximately equal to 1kB (within a 3% error).
1MB is approximately equal to 1mB (within a 5% error).
1GB is approximately equal to 1gB (within a 8% error).
1TB is approximately equal to 1tB (within a 10% error).
The higher the prefix the bigger the relative error while identifying binary and decimal versions.

Evan higher decimal and binary prefixes are used to describe performance of supercomputers:
peta - 1015
exa - 1018
zetta - 1021
yotta - 1024

See also http://en.wikipedia.org/wiki/Binary_prefix .


Originally published in Standard Output, Vol. 1, No. 3, November 2006.