The International System of Units, abbreviated SI (from the original French Système International) is a global standard used to express the magnitudes or quantities of natural phenomena.

What is SI

SI stands for Système International, the name of the original metric system developed by the French. Today, it is used as an abbreviation for the International System of Units, the global standard of measurement. The vast majority of the countries around the world have officially adopted the SI. Since it is the modern form of the metric system, the SI is also referred to as the metric system. SI is the only system that has an official status in almost every country. Even in countries where it may not be used as a primary system of measurement, it is still widely used, particularly in contexts where measurements may be shared on a large or global scale such as international trade, manufacturing, health and safety, security, protection of the environment, and basic science.

The SI is a system comprised of base units and derived units.

Base units

There are 7 SI base units:

Name Quantity
second (s) time
metre (m) length
kilogram (kg) mass
ampere (A) electric current
kelvin (K) thermodynamic temperature
mole (mol) amount of substance
candela (cd) luminous intensity

The precise definitions of the base units are as follows:

Derived unit

SI also allows for an unlimited number of additional units to be derived from SI base units. These are referred to as SI derived units, which are units that can be represented as products of powers of the base units. There are many SI derived units which won't all be listed here. A few examples include degrees Celsius, watts, and volts.

On top of the many derived units of SI, there are also a number of non-SI units that are accepted for use with SI. Some of these include minutes, hours, days, liters, tonnes, and more. Below is a table of some of the named SI derived units.

Name Symbol Quantity SI base unit representation
radian rad plane angle
steradian sr solid angle
hertz Hz frequency
newton N force, weight
pascal PA pressure, stress
joule J energy, work, heat
watt W power, radiant flux
coulomb C electric charge
volt V electric potential, voltage, emf
farad F capacitance
ohm Ω resistance, impedance, reactance
siemens S electrical conductance
weber Wb magnetic flux
tesla T magnetic flux density
henry H inductance
degree Celsius °C temperature relative to 273.15 K
lumen lm luminous flux
lux lx illuminance
becquerel Bq activity referred to a radionuclide
gray Gy absorbed dose of ionising radiation
sievert Sv equivalent dose of ionising radiation
katal kat catalytic activity

SI prefixes

SI prefixes are prefixes such as milli-, centi-, and kilo- that are added to an SI unit to indicate a multiple or submultiple of the unit. Each prefix indicates an integer power of 10. Kilo-, for example, indicates 103 or 1000, while milli- indicates 10-3 or .

A unit cannot have more than one SI prefix. For example, a "millikilogram" cannot exist. There are 24 specified SI prefixes.

SI Prefixes table
Prefix Symbol Factor Factor name
quetta Q 1030 nonillion
ronna R 1027 octillion
yotta Y 1024 septillion
zetta Z 1021 sextillion
exa E 1018 quintillion
peta P 1015 quadrillion
tera T 1012 trillion
giga G 109 billion
mega M 106 million
kilo k 103 thousand
hecto h 102 hundred
deka da 101 ten
100 one
deci d 10-1 tenth
centi c 10-2 hundredth
milli m 10-3 thousandth
micro μ 10-6 millionth
nano n 10-9 billionth
pico p 10-12 trillionth
femto f 10-15 quadrillionth
atto a 10-18 quintillionth
zepto z 10-21 sextillionth
yocto y 10-24 septillionth
ronto r 10-27 octillionth
quecto q 10-30 nonillionth

100 is the base unit, with the exception of the kilogram. The kilogram is the only SI base unit that already has a prefix, and as mentioned, SI units cannot have more than one prefix. SI prefixes are applied to the unit of gram rather than the kilogram.