[[File:SI base units.svg|thumb|right|266px|

{|

|-

! !! Name !! Base quantity

|-

| style="text-align: centre;" | s || second || time

|-

| style="text-align: centre;" | m || metre || length

|-

| style="text-align: centre;" | kg || || mass

|-

| style="text-align: centre;" | A || ampere || electric current

|-

| style="text-align: centre;" | K || kelvin || thermodynamic temperature

|-

| style="text-align: centre;" | mol || mole || amount of substance

|-

| style="text-align: centre;" | cd || candela || luminous intensity

|-

|}

]]

The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre (also spelled meter) for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.

The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology.

The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the metre has the symbol m, but the kelvin has symbol K, because it is named after Lord Kelvin and the ampere with symbol A is named after André-Marie Ampère.

Definitions

On 20 May 2019, as the final act of the 2019 revision of the SI, the BIPM officially introduced the following new definitions, replacing the preceding definitions of the SI base units.

{| class="wikitable"

|+style="font-size:largerfont-weight:bold;"| SI base units

!Name

!style="text-align: center"| Symbol

! Measure

! Post-2019 formal definition

! Historical origin / justification

! Dimensionsymbol

|-

| second

|style="text-align: center"| s

| time

| "The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ∆ν<sub>Cs</sub>, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be when expressed in the unit Hz, which is equal to s<sup>−1</sup>." The 23rd CGPM (2007) decided to postpone any formal change until the next General Conference in 2011.

In a note to the CIPM in October 2009, Ian Mills, the President of the CIPM Consultative Committee – Units (CCU) catalogued the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposed new definition. He urged the CIPM to accept the proposed changes in the definition of the kilogram, ampere, kelvin, and mole so that they are referenced to the values of the fundamental constants, namely the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (N<sub>A</sub>). This approach was approved in 2018, only after measurements of these constants were achieved with sufficient accuracy.

See also

  • International vocabulary of metrology
  • International System of Quantities
  • Non-SI units mentioned in the SI
  • Metric prefix
  • Physical constant

References

  • International Bureau of Weights and Measures
  • National Physical Laboratory
  • NIST – SI