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SI and Data Size Units

SI Prefixes


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The SI specifies a set of metric prefixes (also called SI prefixes). You are probably aware of the term "kilogram" meaning 1000 grams. In this example "kilo" is the SI prefix to the "unit of measure" which is "gram".

Prefix Symbol 10n Decimal Term
yotta Y 1024 1000000000000000000000000 Quadrillion
zetta Z 1021 1000000000000000000000 Trilliard
exa E 1018 1000000000000000000 Trillion
peta P 1015 1000000000000000 Billiard
tera T 1012 1000000000000 Billion
giga G 109 1000000000 Milliard
mega M 106 1000000 Million
kilo k 103 1000 Thousand
hecto h 102 100 Hundred
deca da 101 10 Ten
  100 1 One
deci d 10−1 0.1 Tenth
centi c 10−2 0.01 Hundredth
milli m 10−3 0.001 Thousandth
micro μ 10−6 0.000001 Millionth
nano n 10−9 0.000000001 Milliardth
pico p 10−12 0.000000000001 Billionth
femto f 10−15 0.000000000000001 Billiardth
atto a 10−18 0.000000000000000001 Trillionth
zepto z 10−21 0.000000000000000000001 Trilliardth
yocto y 10−24 0.000000000000000000000001 Quadrillionth


A "byte" is 8 bits. A "kilobyte" is 1000 bytes.
Note: Up until 1999 a kilobyte was the term used to describe 1024 bytes (210 bytes). In 1999 the IEC created the term "kibibyte" to mean 1024 and hence removing the incorrect use of the SI prefix SI when it came to writing Binary lengths.




Data Sizes


IEC Prefix Symbol Value SI Prefix Symbol >1999 <=1999
kibibyte KiB 210 kilobyte kB 103 210
mebibyte MiB 220 megabyte MB 106 220
gibibyte GiB 230 gigabyte GB 109 230
tebibyte TiB 240 terabyte TB 1012 240
pebibyte PiB 250 petabyte PB 1015 250
exbibyte EiB 260 exabyte EB 1018 260
zebibyte ZiB 270 zettabyte ZB 1021 270
yobibyte YiB 280 yottabyte YB 1024 280

One kibibyte is 1024 bytes; no ambiguity now between SI and Binary uses. 1000 bytes is called 1 kilobyte (or 1kB), 1024 bytes is called 1 kibibyte (or 1KiB). Notice how the first 2 characters of the IEC prefix have stayed the same as SI prefixes, but then the characters "bi" have been used to symbolise "binary". An "i" has been added to the symbol to show that it is binary (and the lowercase "k" has been changed to uppercase).

 
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