SI base unit
Name Symbol Measure Post-2019 formal definition Historical origin / justification Dimension symbol
second 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 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 s The day is divided in 24 hours , each hour divided in 60 minutes , each minute divided in 60 seconds . A second is 1 / ( 24 × 60 × 60 ) of the day . Historically this day was defined as the mean solar day ; i.e. , the average time between two successive occurrences of local apparent solar noon T
metre m length The metre , 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 , where the second is defined in terms of ∆ ν Cs 1 / 10 000 000 of the distance from the Earth 's equator to the North Pole measured on the median arc through Paris L
kilogram kg mass 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 when expressed in the unit J s , which is equal to kg m s , where the metre and the second are defined in terms of c and ∆ ν Cs The mass of one litre of water at the temperature of melting ice . A litre is one thousandth of a cubic metre M
ampere A electric current 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 × 10 when expressed in the unit C , which is equal to A s , where the second is defined in terms of ∆ ν Cs The original International Ampere was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate . Compared to the SI ampere , the difference is 0.015% . However , the most recent pre-2019 definition was : The ampere is that constant current which , if maintained in two straight parallel conductors of infinite length , of negligible circular cross-section , and placed one metre apart in vacuum , would produce between these conductors a force equal to 2 × 10 newtons per metre of length . This had the effect of defining the vacuum permeability to be μ 0 = 4 π × 10 H / m or N / A or T ⋅m/A or Wb / ( A⋅m ) or V ⋅ s / ( A ⋅m ) I
kelvin K thermodynamic temperature 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 × 10 when expressed in the unit J K , which is equal to kg m s K , where the kilogram , metre and second are defined in terms of h , c and ∆ ν Cs The Celsius scale : the Kelvin scale uses the degree Celsius for its unit increment , but is a thermodynamic scale ( 0 K is absolute zero ) Θ
mole mol amount of substance The mole , symbol mol , is the SI unit of amount of substance . One mole contains exactly 6.022 140 76 × 10 elementary entities . This number is the fixed numerical value of the Avogadro constant , N A , when expressed in the unit mol 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 Atomic weight or molecular weight divided by the molar mass constant , 1 g/mol N
candela cd luminous intensity 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 × 10 Hz , K cd , to be 683 when expressed in the unit lm W , which is equal to cd sr W , or cd sr kg m s , where the kilogram , metre and second are defined in terms of h , c and ∆ ν Cs The candlepower , which is based on the light emitted from a burning candle of standard properties J
Name Symbol Measure Post-2019 formal definition Historical origin / justification Dimension symbol
Definitions
On 20 May 2019 , as the final act of the 2019 redefinition of the SI base units , the BIPM officially introduced the following new definitions , replacing the preceding definitions of the SI base units .
The SI base units are the standard units of measurement chosen for the seven base quantities chosen by 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 for measurement of length, the kilogram for mass, the ampere for electric current, the kelvin for 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. [citation needed]
 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 (US English: meter) 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. A number of other units, such as the litre (US English: liter), astronomical unit and electronvolt, are not formally part of the SI, but are accepted for use with SI.