The International System of Units (abbreviated **SI** from the French language name *Système international d'unités*) is the world's most widely used system of units. The SI was developed in 1960 and, with few exceptions, is used in every country in the world.

The SI base units for the seven primary quantities are:

Quantity | Name | Symbol |
---|---|---|

Length | metre | m |

Mass | kilogram | kg |

Time | second | s |

Electric current | ampere | A |

Temperature | kelvin | K |

Amount of substance | mole | mol |

Luminous intensity | candela | cd |

Symbols are written in **lower case**, except for symbols derived from the name of a person. For example, the unit of electric current is named after André-Marie Ampère, so its symbol is written "A", whereas the unit itself is written "ampere". The only exception is the **litre**, whose original symbol "l" is unsuitably similar to the numeral "1"; thus it is recommended that "L" be used instead.

Abbreviated symbols should not be pluralized: for example "25 kg", not "25 kgs".

Symbols do not have an appended period (.) unless at the end of a sentence.

A **prefix** may be added to units to produce a **multiple** of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth. The SI main prefixes are as follows:

Name | giga | mega | kilo | hecto | deca | deci | centi | milli | micro | nano |

Symbol | G | M | k | h | da | d | c | m | µ | n |

Factor | 10^{9} | 10^{6} | 10^{3} | 10^{2} | 10^{1} | 10^{-1} | 10^{-2} | 10^{-3} | 10^{-6} | 10^{-9} |