SI
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.