The seven SI base units, which are comprised of:
- Length - meter (m)
- Time - second (s)
- Amount of substance - mole (mole)
- Electric current - ampere (A)
- Temperature - kelvin (K)
- Luminous intensity - candela (cd)
- Mass - kilogram (kg)
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 ΔνCs. |
| 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 m2 s-1, where the meter and the second are defined in terms of c and ΔνCs. |
| 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 ΔνCs, 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 of electric 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 ΔνCs. |
| Unit of thermodynamic 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 m2 s-2 K-1, where the kilogram, meter and second are defined in terms of h, c and ΔνCs. |
| Unit of amount substance | mole | The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 x 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, 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 of luminous 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 1012 Hz, Kcd, 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 s3, where the kilogram, meter and second are defined in terms of h, c and ΔνCs. |
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