"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 SI's base units

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:

### 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
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 () m2.kg.s−3.A−2
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.

#### Candela derived units

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

## Multiples and sub-multiples

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