The name Système International d'Unités (International System of Units) with the international abbreviation SI is a single international language of science and technology first introduced in 1960. SI is a coherent system based on the seven independent physical quantities (base units) and derived quantities (derived units). Note that since 1995 supplementary units have been abandoned and moved into the class of derived SI units.

Physical quantity | quantity symbol | Basic SI Unit Name | Unit Symbol |
---|---|---|---|

length | l, b, d, h, r, s, etc. | metre | m |

mass | m | kilogram | kg |

time | t | second | s |

electric current | I | ampere | A |

thermodynamic temperature | T | kelvin | K |

amount of substance | n | mole | mol |

luminous intensity | I_{v} | candela | cd |

Other physical quantities are derived from the basic units. The derived SI units are obtained by the multiplication, division, integration and differentiation of the basic units without the introduction of any numerical factors. The system of units so derived is said to be coherent.

Quantity | Quantity symbol | SI Unit Name | Unit Symbol | Exp. in SI base units |
---|---|---|---|---|

plane angle | α, β, γ, θ, Φ |
radian | rad | m m^{-1} |

solid angle | ω, Ω | steradian | sr | m^{2} m^{-2} |

The unit kelvin and its symbol K should be used to express both thermodynamic temperature and an interval or a difference of temperature.

In addition to the thermodynamic temperature (symbol *T*) there is also the Celsius (symbol *t*) defined by the equation *t*=*T*-*T*_{0} where *T*_{0}=273.15 K. Celsius temperature is expressed in degree Celsius (symbol C). The unit 'degree Celsius' is equal to the unit 'kelvin', and a temperature interval or a difference of temperature may also be expressed in degrees Celsius. (The word degree and the sign ^{o} must not be used with kelvin or K).

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 particle.

In this definition, it is understood that the carbon 12 atoms are unbound, at rest and in their ground state.

The radian and steradian were classified as supplementary units.

At the time of the introduction of the International System, the question of the nature of these supplementary units was left open. Considering that plane angle is generally expressed as the ratio between two lengths and solid angle as the ratio between an area and the square or a length, it was specified that in the International System the quantities plane angle and solid angle should be considered as dimensionless derived quantities. Therefore, the supplementary units radian and steradian are to be regarded as dimensionless derived units which may be used or omitted in the expressions for derived units.

Since October , 1995, the class of supplementary units as a separate class in the SI has been removed. Thus the SI now consists of only two classes of units: base units and derived units, with the radian and steradian, which were the two supplementary units, moved into the class of SI derived units.

Index of Unit Converters |

To use the measurement conversion calculators simply enter a number value into the desired field and click calculate. All results shown will be equivalent values. Values are given to seven significant figures ( the odd result may show 9's or 0's over running). Values of 10 000 or greater will be displayed in the e-format e.g. 2.3456e7 which equals 2.3456 x 10

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