"The SI is a consistent system of units for use in all aspects of life, including international trade, manufacturing, security, health and safety, protection of the environment, and in the basic science that underpins all of these."
Extract from the SI brochure, CIPM Consultative Committee for Units, 9th edition, 2019
The International System of units (SI)
The SI is made up of 7 base units, supplemented by multiple derived quantities.
The system of quantities which underlies the SI and the equations defining the relationships between these quantities are based on the current description of nature and are known to all scientists, technicians and engineers.
The definitions of SI units are established from a set of seven constants of physics. From the fixed values of these seven constants, expressed in SI units, it is possible to deduce all the units of the system. These seven constants are thus the most essential element in the definition of the entire system of units.
The SI's base units
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:
The metre
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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.
Metre derived units
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
The kilogram
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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.
Kilogram derived units
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
The second
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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.
Second derived units
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
The ampere
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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.
Ampere derived units
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
The kelvin
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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.
Kelvin derived units
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
The mole
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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.
Mole derived units
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
The candela
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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.
