

"The SI is an evolving system that reflects the best current practices in measurement "
Extract from the SI brochure, CIPM's Consultative Committee for Units, 9th edition, 2018
The International System of Units (SI, abbreviated from the French "Système international (des unités)") is the set of units of measurement consisting of the 7 base units associated with the 7 base quantities. It is completed by derived units and prefixes.
The seven basic quantities are: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. The base units are the base on which all the units used to quantitatively express the measured quantities are built. Standards and measurement references are materializations of the measured quantity.
To date, their definitions, are as follows:
Symbol : m
Quantity : length
Definition : The metre 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 the caesium frequency ΔνCs.
Quantity | Unit | Expressed in SI based units |
---|---|---|
area | square meter | m2 |
volume | cubic meter | m3 |
plane angle | radian (rad) | m.m-1 |
solid angle | steradian (sr) | m2.m-2 |
absorbed dose | gray (Gy) | m2.s-2 |
Symbol : kg
Quantity : mass
Definition : The kilogram 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 metre and the second are defined in terms of c and ΔνCs.
Quantity | Unit | Expressed in terms of SI based units |
---|---|---|
force | newton (N) | m.kg.s-2 |
pressure | pascal (Pa) | m-1.kg.s-2 |
electric potential difference | volt (V) | m2.kg.s-3.A-1 |
energy | joule (J) | m2.kg.s-2 |
power, radiant flux | watt (W) | m2.kg.s-3 |
Symbol : s
Quantity : time
Definition : The second is the SI unit of time. It is 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 s−1.
Quantity | Unit | Expressed in terms of SI based units |
---|---|---|
frequency | hertz (Hz) | s-1 |
activity refered to a radionucleide | becquerel (Bq) | s-1 |
dose equivalent | sievert (Sv) | m2.s-2 |
absorbed dose | gray (Gy) | m2.s-2 |
Symbol : A
Quantity : electric current
Definition : The ampere 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 × 10−19 when expressed in the unit C, which is equal to A·s, where the second is defined in terms of ΔνCs.
Quantity | Unit | Expressed in terms of SI based units |
---|---|---|
electric charge | coulomb (C) | s.A |
electric potential difference | volt (V) | m2.kg.s−3.A−1 |
electric resistance | ohm (Ω) | m2.kg.s−3.A−2 |
capacitance | farad (F) | m−2.kg−1.s4.A2 |
inductance | henry (H) | m2.kg.s−2.A−2 |
magnetic flux density | tesla (T) | kg.s−2.A−1 |
Symbol : K
Quantity : thermodynamic temperature
Definition : The kelvin 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 × 10−23 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 ΔνCs.
Quantity | Unit | Expressed in terms of SI based units |
---|---|---|
Celsius temperature t/°C |
degré Celsius (°C) |
K T/K – 273,15 |
thermal conductivity | watt per metre kelvin | m.kg.s-3.K-1 |
surface thermal resistance | square-metre kelvin per watt | kg-1.s3.K |
heat capacity | joule per kelvin | kg.m2.s-2.K-1 |
Symbol : mol
Quantity : amount of substance
Definition : The mole 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.
Quantity | Unit | Expressed in terms of SI based units |
---|---|---|
molar concentration | mole per cubic metre |
mol.m-3 |
catalytic activity | katal (kat) | mol.s-1 |
Symbol : cd
Quantity : luminous intensity
Definition : The candela 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 ΔνCs.
Quantity | Unit | Expressed in terms of SI based units |
---|---|---|
luminous flux | lumen (lm) | cd.sr = m2.m-2.cd |
illuminance | lux (lx) | lm.m-2 = m-2.cd |
Factor | Name | Symbol | Factor | Name | Symbol |
---|---|---|---|---|---|
101 | deca | da | 10-1 | deci | d |
102 | hecto | h | 10-2 | centi | c |
103 | kilo | k | 10-3 | milli | m |
106 | mega | M | 10-6 | micro | µ |
109 | giga | G | 10-9 | nano | n |
1012 | tera | T | 10-12 | pico | p |
1015 | peta | P | 10-15 | femto | f |
1018 | exa | E | 10-18 | atto | a |
1021 | zetta | Z | 10-21 | zepto | z |
1024 | yotta | Y | 10-24 | yocto | y |