SI prefixes

Prof. Dr. Thorsten Jungmann (last updated 2025-08-17)

The International System of Units (‘Système international d’unités’, SI) is the basis of all physical measurements and is therefore also indispensable in electrical engineering. It defines the following prefixes, called SI prefixes, to represent multiples and fractions of the base quantities and units and thus to make it easier to handle very large or very small numerical values in practice. In engineering, the SI prefixes are also referred to as orders of magnitude.

SI prefix	Name	Power	Number word	Number
Y	Yotta	$10^{24}$	septillion	1 000 000 000 000 000 000 000 000
Z	Zetta	$10^{21}$	sextillion	1 000 000 000 000 000 000 000
E	Exa	$10^{18}$	quintillion	1 000 000 000 000 000 000
P	Peta	$10^{15}$	quadrillion	1 000 000 000 000 000
T	Tera	$10^{12}$	trillion	1 000 000 000 000
G	Giga	$10^9$	billion	1 000 000 000
M	Mega	$10^6$	million	1 000 000
k	kilo	$10^3$	thousand	1 000
–	–	$10^0$	one	1
m	milli	$10^{-3}$	thousandth	0.001
µ	micro	$10^{-6}$	millionth	0.000 001
n	nano	$10^{-9}$	billionth	0.000 000 001
p	pico	$10^{-12}$	trillionth	0.000 000 000 001
f	femto	$10^{-15}$	quadrillionth	0.000 000 000 000 001
a	atto	$10^{-18}$	quintillionth	0.000 000 000 000 000 001
z	zepto	$10^{-21}$	sextillionth	0.000 000 000 000 000 000 001
y	yocto	$10^{-24}$	septillionth	0.000 000 000 000 000 000 000 001

Examples:

• The voltage at the output terminals of a sensor, e.g. a microphone, is in the range of a few microvolts ($\mu \mathrm{V}=10^{-6}\mathrm{V}=0.000001\mathrm{V}$).
• The high-voltage lines in the networks of energy suppliers carry several hundred kilovolts ($\mathrm{kV}=10^3\mathrm{V}=1000\mathrm{V}$).
• Capacitors have capacitances in the order of picofarads ($\mathrm{pF}=10^{-12}\mathrm{F}=0.000000000001\mathrm{F}$) , nanofarads ($\mathrm{nF}=10^{-9}\mathrm{F}=0.000000001\mathrm{F}$) or microfarads ($\mu \mathrm{F}=10^{-6}\mathrm{F}=0.000001\mathrm{F}$).
• The contact resistance of a commercially available light switch is in the range between 10 and 200 milliohms ($\mathrm{m}\Omega =10^{-3}\Omega =0.001\Omega$) .
• The frequency of wireless networks (WiFi) is in the range of a few gigahertz ($\mathrm{GHz}=10^9\mathrm{Hz}=1000000000\mathrm{Hz}$).