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Fundamentals of SI Units

The foundation of modern metrology is the International System of Units (SI), which is used internationally to define the fundamental units of measurement. These are used to derive other units and are used to define other measurement units, such as may be used locally.

The SI consists of 28 units:

These are defined in the tables below for reference.

Base Units

ParameterSI UnitAbbreviationDefinition
LengthmetremThe distance travelled by light in a vacuum in 1/299,792,458 s.
MasskilogramkgThe mass of a cylinder of platinum iridium alloy kept in Paris, France.
TimesecondsThe duration of 9,192,631,770 cycles of radiation corresponding to a particular transition of the caesium-133 atom.
Electric Current ampereAThe current producing a force of 2*10-7 newtons per metre of length between two long wires 1 m apart in space.
Thermodynamic Temperature kelvinK1/273.16 of the thermodynamic triple point of water.
Luminous Intensity candelacdThe luminous intensity in a given direction of a source that emits monochromatic radiation at a frequency of 540*1012 hertz, with a radiant intensity in that direction of 1/683 watts per steradian.
Amount of Substance molemThe amount of substance in a system that contains as many elementary items as there are atoms in 0.012 kg of carbon 12.

Supplementary Units

ParameterSI UnitAbbreviationDefinition
Plane AnglesradianradA plane angle with vertex at the center of a circle that is subtended by an arc equal in length to the radius.
Solid Angles steradiansrThe solid angle with vertex at the center of a sphere that is subtended by an area of the spherical circle equal to that of a square with sides equal in length to the radius.

Derivative Units

ParameterSI UnitAbbreviationDefinition
Frequency hertzHz1/s
Force newtonNkg*m/s²
Pressure pascalPaN/m²
Work or Energy jouleJN*m
Power wattWJ/s
Electric Potential voltVW/A
Electric Resistance ohmohmV/A
Quantity of Charge coulombCA*s
Electric Capacitance faradFC/V
Conductance siemensSA/V
Magnetic FluxweberWbV*s
Magnetic Flux Density teslaTWb/²
Inductance henryHWb/A
Celsius Temperaturedegree°CK
Luminous Flux lumenlmcd*sr
Illuminance luxlxlm/m²
Activity becquerelBq1/s
Absorbed Dose grayGyJ/kg
Dose Equivalent sievertSvm²/s²