The recommended practical system of units of measurement is the International System of Units (Système International d'Unités, with the international abbreviation SI).

The SI is defined by the SI Brochure, which is published by the BIPM.

This SI consists of a set of base units, prefixes and derived units, as described in these pages:

• The SI base units are a choice of seven well-defined units which by convention are regarded as dimensionally independent: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela.


• Derived units are formed by combining the base units according to the algebraic relations linking the corresponding quantities. The names and symbols of some of the units thus formed can be replaced by special names and symbols which can themselves be used to form expressions and symbols of other derived units.

The SI is not static but evolves to match the world's increasingly demanding requirements for measurement. Currently much work is under way related to the intended future revision of the SI.

The SI base units are a choice of seven well-defined units which by convention are regarded as dimensionally independent:

metre, m
	The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. [ SI Brochure ]		Practical realization Evolution of the definition CCL
kilogram, kg
	The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. [ SI Brochure ]		Practical realization The name "kilogram" Work at the BIPM CCM
second, s
	The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. [ SI Brochure ]		Practical realization Secondary representations Work at the BIPM CCTF
ampere, A
	The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length. [ SI Brochure ]		Practical realization Work at the BIPM CCEM
kelvin, K
	The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. [ SI Brochure ]		Practical realization History CCT
mole, mol
	The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. [ SI Brochure ]		Practical realization Work at the BIPM CCQM
candela, cd
	The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. [ SI Brochure ]		Practical realization CCPR

All other SI units can be derived from these, by multiplying together different powers of the base units.

Unique publications for international metrology:

• SI brochure
• Mises en pratique: Instructions for practical realization of the SI units
• Guides in Metrology: GUM and VIM
• Metrologia: the international journal of metrology
• Circular T
• List of recommended standard frequencies
• IERS Conventions Center: the reference systems and procedures realized by the International Earth Rotation and Reference Systems Service (IERS)

Decimal multiples and submultiples of SI units can be written using the SI prefixes listed in the table below:

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

For full details please refer to Chapter 3 of the SI Brochure.