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 ΔνCs, 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 × 1023 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 NA, 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 × 1012 Hz, to be 683 when expressed in the unit \(\frac {s^3⋅cd⋅sr}{kg⋅m^2}\). |