The International System of Units (SI), is a system of measurement used throughout most of the world. It is the modern form of the metric system, and as part of a push to standardize measurement globally, 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 usually recognized as an official system of measurement, and is particularly used in contexts where measurements may be shared on a large or global scale; in such cases, standardization is important.

An example of this would be a number of countries working on some project. If each country were using a different system of measurement, it would likely cause a lot of confusion and would make it difficult for the countries to work together.

SI 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

SI derived units and accepted non-SI units

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.

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 20 specified SI prefixes.

Prefix Power of 10
yotta 1024
zetta 1021
exa 1018
peta 1015
tera 1012
giga 109
mega 106
kilo 103
hecto 102
deca 101
- 100
deci 10-1
centi 10-2
milli 10-3
micro 10-6
nano 10-9
pico 10-12
femto 10-15
atto 10-18
zepto 10-21
yocto 10-24

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.