SI prefixes

SI prefixes are used to apply a scale factor to an SI unit of measurement. The prefix is used before a unit of measurement to form a decimal multiple or submultiple of the unit. Over the years, the list of prefixes has been extended, and these prefixes are currently recognized[1]:

Symbol	Name	Factor	Multiple
Y	yotta	1024	one septillion
Z	zetta	1021	one sextillion
E	exa	1018	one quintillion
P	peta	1015	one quadrillion
T	tera	1012	one trillion
G	giga	109	one billion
M	mega	106	one million
k	kilo	103	one thousand
h	hecto	102	one hundred
da	deka or deca	101	ten
d	deci	10-1	one tenth
c	centi	10-2	one hundredth
m	milli	10-3	one thousandth
µ	micro	10-6	one millionth
n	nano	10-9	one billionth
p	pico	10-12	one trillionth
f	femto	10-15	one quadrillionth
a	atto	10-18	one quintillionth
z	zepto	10-21	one sextillionth
y	yocto	10-24	one septillionth

Examples

The prefix is written before the unit, joined without a space or punctuation. Here are some examples:

• millimeter (mm) - one thousandth of a meter
• microgram (µg) - one millionth of a gram
• kilometer (km) - one thousand meters
• megavolt (MV) - one million volts

Notes

• The SI base unit of mass is the kilogram; however the SI prefixes are applied to the gram.
• Awkward paired vowels are replaced by a single vowel. One million ohms is a megohm, not a megaohm.
• The SI prefixes centi, deci, deka and hecto are rarely used, with the exception of the centilitre (for wine), the centimeter, and the hectare - and even those prefixes are not normally used within the fields of science and engineering.
• The prefix myria, corresponding to a factor of ten thousand, is obsolete.
• The symbol for the prefix micro is the Greek letter µ, but is sometimes written as u for convenience.
• The prefix deka is officially preferred in the United States, and deca elsewhere.
  

Prefixes for binary multiples

SI prefixes are sometimes loosely used to describe binary multiples. For example, 1024 bytes is sometimes called a kilobyte. This usage, although common, is incorrect because the SI prefixes represent decimal multiples. To resolve this confusion, the IEC has published a list of prefixes for binary multiples. So, for example, 1024 bytes becomes a kibibyte (kiB).