Become familiar with the seven defining constants of the SI.

**Definitions of the SI base units**

Unit of length |
meter |
The meter, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s^{-1}, where the second is defined in terms of Δν_{Cs}. |

Unit of mass |
kilogram |
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10^{-34} when expressed in the unit J s, which is equal to kg m^{2} s^{-1}, where the meter and the second are defined in terms of c and Δν_{Cs}. |

Unit of time |
second |
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency Δν_{Cs}, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^{-1}. |

Unit ofelectric current |
ampere |
The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 x 10^{-19} when expressed in the unit C, which is equal to A s, where the second is defined in terms of Δν_{Cs}. |

Unit ofthermodynamic temperature |
kelvin |
The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 x 10^{-23} when expressed in the unit J K^{-1}, which is equal to kg m^{2} s^{-2} K^{-1}, where the kilogram, meter and second are defined in terms of h, c and Δν_{Cs}. |

Unit ofamount of substance |
mole |
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 x 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, N_{A}, when expressed in the unit mol^{-1} and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. |

Unit ofluminous intensity |
candela |
The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 x 10^{12} Hz, K_{cd}, to be 683 when expressed in the unit lm W^{-1}, which is equal to cd sr W^{-1}, or cd sr kg^{-1} m^{-2} s^{3}, where the kilogram, meter and second are defined in terms of h, c and Δν_{Cs}. |

**Go to SI Units Background** **or **SI base units