# Metric units

Metric units are units based on the metre, gram or second and decimal (power of ten) multiples or sub-multiples of these. The most widely used examples are the units of the International System of Units (SI). By extension they include units of electromagnetism from the CGS and SI units systems, and other units for which use of SI prefixes has become the norm. Other unit systems using metric units include:

## Metric units that are part of the SI

The first group of metric units are those that are at present defined as units within the International System of Units (SI). In its most restrictive interpretation, this is what may be meant when the term metric unit is used.

The SI defines 29 named units and associated symbols:

• The second (s) is the unit of time.
• The metre (m) is the unit of length.
• The kilogram (kg) is the unit of mass.
• The ampere (A) is the unit of electric current.
• The kelvin (K) is the unit of thermodynamic temperature.
• The mole (mol) is the unit of amount of substance.
• The candela (cd) is the unit of luminous intensity.
• The hertz (Hz) is equal to one reciprocal second (1 s−1).
• The steradian (sr) is equal to one (1).
• The newton (N) is equal to one kilogram-metre per second squared (1 kg⋅m⋅s−2).
• The pascal (Pa) is equal to one newton per square metre (1 N/m2).
• The joule (J) is equal to one newton metre (1 N⋅m).
• The watt (W) is equal to one joule per second (1 J/s).
• The coulomb (C) is equal to one ampere second (1 A⋅s).
• The volt (V) is equal to one joule per coulomb (1 J/C).
• The weber (Wb) is one volt second (1 V⋅s).
• The tesla (T) is one weber per square metre (1 Wb/m2).
• The farad (F) is equal to one coulomb per volt (1 C/V).
• The ohm (Ω) is equal to one volt per ampere (1 V/A).
• The siemens (S) is equal to one ampere per volt (1 A/V).
• The henry (H) is one volt second per ampere (1 V⋅s/A).
• The degree Celsius (°C) is equal to one kelvin (1 K).
• The lumen (lm) is equal to one candela steradian (1 cd⋅sr).
• The lux (lx) is equal to on lumen per square metre (1 lm/m2).
• The becquerel (Bq) is equal to one reciprocal second (1 s−1).
• The gray (Gy) is equal to one joule per kilogram (1 J/kg).
• The sievert (Sv) is equal to one joule per kilogram (1 J/kg).
• The katal (kat) is equal to one mole per second (1 mol/s).

Further, there are twenty metric prefixes that can be combined with any of these units (with the gram (g) in the case of the kilogram) to form further units that are decimal multiples or submultiples of these to form further SI units.

## Metric units that are not part of the SI

Metric units that are not part of the SI include

### Spatial

#### Area

• The shed is equal to 10−24 barns (10−52 m2).
• The outhouse is equal to 10−6 barns (100 am2 = 10−34 m2).
• The barn (b) is equal to one hundred femtometres squared (100 fm2 = 10−28 m2).
• The are (a) is equal to one hundred metres squared (100 m2).
• The decare (daa) is equal to one thousand metres squared (1000 m2).
• The dunam is equal to one thousand metres squared (1000 m2).
• The stremma is equal to one thousand metres squared (1000 m2).
• The hectare (ha) is equal to ten thousand metres squared (10,000 m2).

### Temporal

#### Other

• The eotvos (E) is equal to 10−9 Gal/cm (10−9 s−2).

### Combined spatial and temporal

For the stokes, a unit of dimension L2 T−1, see § Viscosity.

#### Speed

• The benz is equal to one metre per second (1 m/s).

#### Acceleration

• The gal (Gal) is a CGS unit of acceleration equal to one centimetre per second squared (1 cm/s2).
• The leo is equal to 10 m⋅s−2.

### Mechanics

#### Mass

• The undecimogramme is equal to ten picograms (10 pg).
• The gamma (γ) is equal to one microgram (1 μg).
• The carat (ct) is equal to 200 mg.
• The grave is equal to one kilogram (1 kg).
• The normal litre (NL) is a unit of mass used in gas flow equal to the product of 1 litre and the density of the gas in specified standard conditions. 
• The hyl is equal to 9.80665 kg (or 9.80665 g).
• The metric quintal (q) is equal to one hundred kilograms (100 kg).
• The tonne (t) is an MTS unit equal to one megagram (1 Mg).

#### Linear mass density

• The tex (tex) is equal to one gram per kilometre (1 g/km).
• The number metric (Nm) is equal to 1000 metres per kilogram (1000 m/kg).

#### Energy

• The electronvolt (eV) is equal to 1.602176634×10−19 J = 160.2176634 zJ.
• The erg (erg) is a CGS unit equal to one dyne centimeter (1 dyn cm = 100 nJ).
• The calorie (cal) is 4.184 J.
• The watt hour (W h) is equal to 3.6 kJ.
• The foe is equal to 1044 joules.

#### Viscosity

• The poise (P) is a unit of dynamic viscosity equal to one gram per (centimeter second) or one hundred grams per (metre second) (1 g⋅cm−1⋅s−1 = 100 g⋅m−1 s−1).
• The poiseuille is a unit of dynamic viscosity equal to one pascal-second (1 Pa⋅s).
• The stokes (St) is a unit of kinematic viscosity equal to one centimetre squared per second or one hundred millimetres squared per second (1 cm2/s = 1 P⋅g⋅cm−3 = 100 mm2/s).

### Electromagnetism

#### Electricity

##### Charge
• The statcoulomb (statC) is a unit of electric charge equal to 1 dyn1/2⋅cm, corresponding to approximately 3.33564×10−10 C.
• The abcoulomb (abC) is a CGS unit of electric charge corresponding to ten coulombs (10 C).
• The ampere hour is a unit of electric charge equal to 3.6 kC.

#### Light

• The nox (nx) is a unit of illuminance equal to 1 millilux (1 mlx).
• The phot (ph) is a unit of illuminance equal to 10 kilolux (10 klx).
• The nit (nt) is a unit of luminance equal to one candela per metre squared (1 cd⋅m−2).
• The stilb (sb) is a unit of luminance equal to ten kilocandelas per metre squared (10 kcd⋅m−2).
• The einstein (E) is a unit of amount of photons, equal to one mole (1 mol) of photons.