Finite quantity that is used as the base for measurements of other quantities of the same type.

The seven base units of the International System of Units are:

Quantity |
Units |
Symbol |
Description |

Length |
Metre |
m |
Length 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 \(\frac{m}{s}\). |

Mass |
Kilogram |
kg |
Mass defined ty taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10^{−34} when expressed in the unit \(J\cdot s\) , which is equal to \(\frac{kg \cdot m^2}{s}\). |

Time |
Second |
s |
Duration defined by taking the fixed numerical value of the caesium frequency Δν_{Cs}, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to \(\frac{1}{s}\). |

Intensity of an electric current |
Ampere |
A |
Intensity of electrical current defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634×10^{−19} when expressed in the unit C, which is equal to \(A \cdot s\). |

Temperature |
Kelvin |
K |
Unit of thermodynamic temperature defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10^{−23} when expressed in the unit \(\frac{J}{K}\), which is equal to \(\frac{kg \cdot m^2}{K \cdot s^2}\). |

Quantity of matter |
Mole |
mol |
Quantity of matter corresponding to contains exactly 6.022 140 76 × 10^{23} elementary entities, which may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. This number is the fixed numerical value of the Avogadro constant N_{A}, when expressed in the unit \(\frac{1}{mol}\) and is called the Avogadro number. |

Light intensity |
Candela |
cd |
Light intensity in a given direction, defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10^{12} Hz, to be 683 when expressed in the unit \(\frac {s^3⋅cd⋅sr}{kg⋅m^2}\). |