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"*The SI is an evolving system that reflects the best current practices in measurement *"

Extract from the SI brochure, CIPM's Consultative Committee for Units, 8th edition, 2006

## The SI's base units

The International System of Units (SI, abbreviated from the French "*Système international (des unités)*") is the set of units of measurement consisting of the 7 base units associated with the 7 base quantities. It is completed by derived units and prefixes.

The seven basic quantities are: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. The base units are the base on which all the units used to quantitatively express the measured quantities are built. Standards and measurement references are materializations of the measured quantity.

To date, their definitions, are as follows:

### The metre

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**Symbol** : m

**Quantity** : length

**Definition** : The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

#### Metre derived units

Quantity | Unit | Expressed in SI based units |
---|---|---|

area | square meter | m^{2} |

volume | cubic meter | m^{3} |

plane angle | radian (rad) | m.m^{-1} |

solid angle | steradian (sr) | m^{2}.m^{-2} |

absorbed dose | gray (Gy) | m^{2}.s^{-2} |

### The kilogram

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**Symbol** : kg

**Quantity** : mass

**Definition** : The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

#### Kilogram derived units

Quantity | Unit | Expressed in terms of SI based units |
---|---|---|

force | newton (N) | m.kg.s^{-2} |

pressure | pascal (Pa) | m^{-1}.kg.s^{-2} |

electric potential difference | volt (V) | m^{2}.kg.s^{-}^{3}.A^{-}^{1} |

energy | joule (J) | m^{2}.kg.s^{-2} |

power, radiant flux | watt (W) | m^{2}.kg.s^{-3} |

### The second

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**Symbol** : s

**Quantity** : time

**Definition** : The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

#### Second derived units

Quantity | Unit | Expressed in terms of SI based units |
---|---|---|

frequency | hertz (Hz) | s^{-1} |

activity refered to a radionucleide | becquerel (Bq) | s^{-1} |

dose equivalent | sievert (Sv) | m ^{2}.s^{-2} |

absorbed dose | gray (Gy) | m^{2}.s^{-2} |

### The ampere

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**Symbol** : A

**Quantity** : electric current

**Definition** : The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10^{–7} newton per metre of length.

#### Ampere derived units

Quantity | Unit | Expressed in terms of SI based units |
---|---|---|

electric charge | coulomb (C) | s.A |

electric potential difference | volt (V) | m^{2}.kg.s^{−3}.A^{−1} |

electric resistance | ohm (Ω) | m^{2}.kg.s^{−3}.A^{−2} |

capacitance | farad (F) | m^{−2}.kg^{−1}.s^{4}.A^{2} |

inductance | henry (H) | m^{2}.kg.s^{−2}.A^{−2} |

magnetic flux density | tesla (T) | kg.s^{−2}.A^{−1} |

### The kelvin

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**Symbol** : K

**Quantity** : thermodynamic temperature

**Definition** : The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

#### Kelvin derived units

Quantity | Unit | Expressed in terms of SI based units |
---|---|---|

Celsius temperature t/°C |
degré Celsius (°C) |
K T/K – 273,15 |

thermal conductivity | watt per metre kelvin | m.kg.s^{-3}.K^{-1} |

surface thermal resistance | square-metre kelvin per watt | kg^{-1}.s^{3}.K |

heat capacity | joule per kelvin | kg.m^{2}.s^{-2}.K^{-1} |

### The mole

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**Symbol** : mol

**Quantity** : amount of substance

**Definition** : 1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol".

2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

#### Mole derived units

Quantity | Unit | Expressed in terms of SI based units |
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molar concentration | mole per cubic metre |
mol.m |

catalytic activity | katal (kat) | mol.s^{-1} |

### The candela

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**Symbol** : cd

**Quantity** : luminous intensity

**Definition** : The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^{12} hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

#### Candela derived units

Quantity | Unit | Expressed in terms of SI based units |
---|---|---|

luminous flux | lumen (lm) | cd^{.}sr = m^{2}.m^{-2}.cd |

illuminance | lux (lx) | lm^{.}m^{-2} = m^{-2}.cd |

## Multiples and sub-multiples

Factor | Name | Symbol | Factor | Name | Symbol |
---|---|---|---|---|---|

10^{1} |
deca | da | 10^{-1} |
deci | d |

10^{2} |
hecto | h | 10^{-2} |
centi | c |

10^{3} |
kilo | k | 10^{-3} |
milli | m |

10^{6} |
mega | M | 10^{-6} |
micro | µ |

10^{9} |
giga | G | 10^{-9} |
nano | n |

10^{12} |
tera | T | 10^{-12} |
pico | p |

10^{15} |
peta | P | 10^{-15} |
femto | f |

10^{18} |
exa | E | 10^{-18} |
atto | a |

10^{21} |
zetta | Z | 10^{-21} |
zepto | z |

10^{24} |
yotta | Y | 10^{-24} |
yocto | y |