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Units of Measurement

SI Base Units

The name Système International d'Unités (International System of Units)) with the international abbreviation SI is a single international language of science and technology first introduced in 1960. SI is a coherent system based on the seven independent physical quantities (base units) and derived quantities (derived units). Note that since 1995 supplementary units have been abandoned and moved into the class of derived SI units.

Basic SI Units

Physical quantity	quantity symbol	Basic SI Unit Name	Unit Symbol
length	l,b,d,h,r,s,etc.	metre	m
mass	m	kilogram	kg
time	t	second	s
electric current	I	ampere	A
thermodynamic temperature	T	kelvin	K
amount of substance	n	mole	mol
luminous intensity	I_v	candela	cd

Table 1. SI base units.
Other physical quantities are derived from the basic units. The derived SI units are obtained by the multiplication, division, integration and differentiation of the basic units without the introduction of any numerical factors. The system of units so derived is said to be coherent.

Supplementary Dimensionless SI Units

Physical Quantity	Quantity symbol	SI Unit Name	Unit Symbol	Expression in SI base units
plane angle	α , β , γ , θ , Φ	radian	rad	m m^-1
solid angle	ω , Ω	steradian	sr	m² m^-2

Table 2. SI supplementary units. (classification removed, see notes)

Definitions of the SI Base Units

Length: 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.

Mass: kilogram (kg)

The kilogram is equal to the mass of the international prototype of the kilogram: a piece of platinum-iridium alloy kept at the International Bureau of Weights and Measures, Sévres, France.

Time: 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 cesium-133 atom.

Electric current: 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 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10^-7 newton per metre of length.

Thermodynamic temperature: kelvin (K)

The kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water.

The unit kelvin and its symbol K should be used to express both thermodynamic temperature and an interval or a difference of temperature.

In addition to the thermodynamic temperature (symbol T) there is also the Celsius (symbol t) defined by the equation t=T-T₀ where T₀=273.15 K. Celsius temperature is expressed in degree Celsius (symbol C). The unit 'degree Celsius' is equal to the unit 'kelvin', and a temperature interval or a difference of temperature may also be expressed in degrees Celsius. (The word degree and the sign ^o must not be used with kelvin or K).

Amount of substance: 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 particle.

In this definition, it is understood that the carbon 12 atoms are unbound, at rest and in their ground state.

Luminous intensity: 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.

Definition of Supplementary SI Units

Plane angle: radian (rad) and solid angle: steradian (sr)

The radian and steradian were classified as supplementary units.

At the time of the introduction of the International System, the question of the nature of these supplementary units was left open. Considering that plane angle is generally expressed as the ratio between two lengths and solid angle as the ratio between an area and the square or a length, it was specified that in the International System the quantities plane angle and solid angle should be considered as dimensionless derived quantities. Therefore, the supplementary units radian and steradian are to be regarded as dimensionless derived units which may be used or omitted in the expressions for derived units.

Since October , 1995, the class of supplementary units as a separate class in the SI has been removed. Thus the SI now consists of only two classes of units: base units and derived units, with the radian and steradian, which were the two supplementary units, moved into the class of SI derived units.

Si Units:

	SI Base Units
	SI Derived Units
		expressed with special names
		expressed in terms of SI base units
		expressed in terms in terms of SI units with special names
	SI Unit Prefixes
	Non-SI Units
		accepted by SI
		currently accepted by SI
		derived CGS units with special names
		other non-SI units
	Constants
	Glossary of Units

CONVERSION CALCULATORS

Convert between Different Units of Measurement

Index of Unit Converters

ACCELERATION	ANGLE	AREA	AREA DENSITY
DENSITY	DISTANCE	DYNAMIC VISCOSITY	ENERGY
ENERGY DENSITY	ENTROPY	FEED RATE	FLOW RATE
FORCE	FUEL CONSUMPTION	HEAT CAPACITY	HEAT FLUX DENSITY
HEAT TRANSFER COEFFICIENT	IRRADIANCE	KINEMATIC VISCOSITY	LENGTH
MASS	POWER	PRESSURE	SPECIFIC ENERGY
SPECIFIC ENTHALPY	SPECIFIC ENTROPY	SPECIFIC HEAT CAPACITY	SPECIFIC VOLUME
SPEED	TEMPERATURE	THERMAL CONDUCTIVITY	TIME
TORQUE	VELOCITY	VOLUME	VOLUMETRIC HEAT CAPACITY
Unit Prefixes	Plasma Gas Flow	Hardness MPa - GPa	Computing Bits and Bytes

To use the measurement conversion calculators simply enter a number value into the desired field and click calculate. All results shown will be equivalent values. Values are given to seven significant figures ( the odd result may show 9's or 0's over running). Values of 10 000 or greater will be displayed in the e-format e.g. 2.3456e7 which equals 2.3456 x 10⁷ or 23 456 000. Values lower than 0.001 will be displayed in the e-format e.g. 2.3456e-5 which equals 2.3456 x 10^-5 or 0.000 023456. The calculators require that your browser has java script enabled. View all measurement unit conversion calculators on the same page (may not work with all browsers; requires Iframe).

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