Gordon England Surface Engineering Forum

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 mkilogram kg
time tsecond s
electric current Iampere A
thermodynamic temperature Tkelvin K
amount of substance nmole mol
luminous intensity Ivcandela 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 NameUnit Symbol Expression in SI base units
plane angle α , β , γ , θ , Φ radian rad m m-1
solid angle ω , Ωsteradian 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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