Name |
Symbol |
Measure |
Post-2019 formal definition[1] |
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, ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1."[1] |
The day is divided into 24 hours, each hour divided into 60 minutes, each minute divided into 60 seconds. A second is 1 / (24 × 60 × 60) of the day. Historically, a 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 299792458 when expressed in the unit m s−1, where the second is defined in terms of ∆νCs."[1] |
1 / 10000000 of the distance from the Earth's equator to the North Pole measured on the meridian 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 6.62607015×10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆νCs."[1] |
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 1.602176634×10−19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆νCs."[1] |
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. |
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 1.380649×10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ∆νCs."[1] |
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 × 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."[1] |
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 540×1012 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, metre and second are defined in terms of h, c and ∆νCs."[1] |
The candlepower, which is based on the light emitted from a burning candle of standard properties. |
J |