### International System of Units

International System of Units is a convention that defines seven standard units for the measurement of seven physical parameters shown in the table below. Briefly, the units are referrred to as SI units. SI is the acronym for Système international in French. The second table below shows 22 units derived from the seven units.

Parameter | Name | Symbol |
---|---|---|

time | second | s |

length | metre | m |

mass | kilogram | kg |

electric current | ampere | A |

thermodynamic temperature | kelvin | K |

amount of substance | mole | mol |

luminous intensity | candela | cd |

Click a column heading to resort the Table.

Quantity | Name | Symbol | In SI base units^{a} |
---|---|---|---|

plane angle | radian^{b} | rad | m/m |

solid angle | steradian^{c} | sr | m^{2}/m^{2} |

frequency | hertz^{d} | Hz | s^{−1} |

force, weight | newton | N | kg⋅m⋅s^{−2} |

pressure, stress | pascal | Pa | kg⋅m^{−1}⋅s^{−2} |

energy, work, heat | joule | J | kg⋅m^{2}⋅s^{−2} |

power, radiant flux | watt | W | kg⋅m^{2}⋅s^{−3} |

electric charge | coulomb | C | s⋅A |

electric potential difference^{e} | volt | V | kg⋅m^{2}⋅s^{−3}⋅A^{−1} |

capacitance | farad | F | kg^{−1}⋅m^{−2}⋅s^{4}⋅A^{2} |

electric resistance | ohm | Ω | kg⋅m^{2}⋅s^{−3}⋅A^{−2} |

electric conductance | siemens | S | kg^{−1}⋅m^{−2}⋅s^{3}⋅A^{2} |

magnetic flux | weber | Wb | kg⋅m^{2}⋅s^{−2}⋅A^{−1} |

magnetic flux density | tesla | T | kg⋅s^{−2}⋅A^{−1} |

inductance | henry | H | kg⋅m^{2}⋅s^{−2}⋅A^{−2} |

Celsius temperature | degree Celsius^{f} | °C | K |

luminous flux | lumen | lm | cd⋅sr |

illuminance | lux | lx | cd⋅sr⋅m^{−2} |

radioactivity (decays per unit time)^{d,h} | becquerel | Bq | s^{−1} |

absorbed dose (of ionizing radiation) | gray | Gy | m^{2}⋅s^{−2} |

dose equivalent (of ionizing radiation) | sievert^{i} | Sv | m^{2}⋅s^{−2} |

catalytic activity | katal | kat | mol⋅s^{−1} |

**Notes for the Table:**

^{a)} The order of symbols for base units in this Table is different from that in the 8th edition following decision by the CCU at its 21st meeting (2013) to return to the original order in Resolution 12 of th 11th CGPM (1960) in which newton was written kg m s^{−2}, the joule as kg m^{2} s^{−2} and J s as kg m^{−2} s^{−1}. The intention was to reflect the underlying physics of the corresponding quantity equations although for some more complex derived units this may not be possible.

^{b)} The radian is the coherent unit for plane angle. One radian is the angle subtended at the centre of a circle by an arc that is equal in length to the radius. It is also the unit for phase angle. For periodic phenomena, the phase angle increases by 2π rad in one period. The radian was formerly an SI supplementary unit, but this category was abolished in 1995.

^{c)} The steradian is the coherent unit for solid angle. One steradian is the solid angle subtended at the centre of a sphere by an area of the surface that is equal to the squared radius. Like the radian, the steradian was formerly an SI supplementary unit.

^{d)} The hertz shall only be used for periodic phenomena and the becquerel shall only be used for stochastic processes in activity referred to a radionuclide.

^{e)}Electric potential difference is also called “voltage” in many countries, as well as “electric tension” or simply “tension” in some countries.

^{f)}The degree Celsius is used to express Celsius temperatures. The numerical value of a temperature difference or temperature interval is the same when expressed in either degrees Celsius or in kelvin.

^{g)}In photometry the name steradian and the symbol sr are usually retained in expressions for units

^{h)}Activity referred to a radionuclide is sometimes incorrectly called radioactivity.

^{i)}See CIPM Recommendation 2 on the use of the sievert (PV, 2002, 70, 205).

The tables above are based on Tables 2 and 4 from the Official SI Brochure.