International System of Units (SI)

The international system of units is the most widely used system of measurement, and is the modern form of the metric system. At its core, this system is built on seven primary units. This system specifies twenty prefixes to the unit symbols and name to state multiples and fractions of each unit.

The international system of units was introduced to the public in 1960 as a result to earlier research that began in 1948. SI is based on the meter-kilogram second system of units (MKS). SI was created to evolve as time progressed forward. It was intended that units and prefixes were to be created and unit definitions to be modified, on an international level, as the technology of the measuring tools we use on a daily basis continues to become more advanced and more precise. An excellent example of this evolving system would be the 24th and 25th General Conferences on Weights and Measures (CGPM) that took place in 2011 and 2014 where a proposal was introduced to alter the definition of “kilogram”. This was brought up because some began to believe that a kilogram was an invariant of nature rather than a measurement of mass.

As mentioned in the previous section, there was an incredible lack of coordination amongst the various professions in a number of areas across the world. To create unity, SI was born. The CGPM, established by the Meter Convention of 1875, took on the mission of bringing together numerous international organizations to agree upon the definitions and standards of the proposed new system. They were also brought together to agree upon the rules about how this new system would be presented and accepted around the world.

In theory, SI can be used for any type of physical measurement. Despite this, the fact could not be ignored that non-SI units of measurement would still appear in the technical, scientific and commercial literature for the foreseeable future. Furthermore, certain units of measurement have been engrained so deeply in history and certain cultures that they will continue to be used for a long time to come. The CIPM has categorized these units and incorporated them into the SI Brochure.

Below is the international system of units.

Base Units

AmountUnit nameUnit symbolDefinition
LengthMetermThe distance traveled by light in a vacuum in 1/299792458 second.
WeightKilogramkgThis is the unit for weight. The mass of the international prototype kilogram.
TimeSecondsThe duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
CurrentAmpereAThe 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 2×10-7 newtons per meter of length.
Thermodynamic temperatureKelvinK1/273.16 of the thermodynamic temperature of the triple point of water.
Substance amountMolemolThe amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. (Limited to objects with clarified composition.) Elementary entities are subatomic particles that compose matter and energy.
LuminosityCandelacdThe luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Supplementary Units

AmountUnit nameUnit symbolDefinition
Plane angleRadianradRadian describes the plane angle subtended by an arc of a circle with the same length as the radius of that circle corresponds to an angle of 1 radian.
Solid angleSteradiansrA steradian is a solid angle at the center of a sphere subtending a section on the surface equal in area to the square of the radius of the sphere.

Derived Units

Derived units are a combination of base units and supplementary units and the mathematical symbols of multiplication and division.

AmountUnit nameUnit symbol
AreaSquare meterm2
VolumeCubic meterm3
SpeedMeter per secondm/s
AccelerationMeter per second squaredm/s2
WavenumberReciprocal meterm-1
DensityKilogram per cubic meterkg/m3
Current densityAmpere per square meterA/m2
Magnetic field strengthAmpere per meterA/m
Concentration (of amount of substance)Mole per cubic metermol/m3
Specific volumeCubic meter per kilogramm3/kg
LuminanceCandela per square metercd/m2

Some derived units are given unique names.

AmountUnit nameUnit symbolComposition
FrequencyHertzHz1Hz=1s-1
ForceNewtonN1N=1kg・m/s2
Pressure, stressPascalPa1Pa=1N/m2
Energy, work, amount of heatJouleJ1J=1N・m
Power, radiant fluxWattWW=1J/s
Electric charge, amount of electricityCoulombC1C=1A・s
Electric potential/electric potential difference, voltage, electromotive forceVoltV1V=1J/C
Resistance (electrical)OhmΩ1Ω=1V/A
Conductance (electrical)SiemensS1S=1Ω-1
MagneticWeberWb1Wb=1V・s
Magnetic flux density, magnetic inductionTeslaT1T=1Wb/m2
InductanceHenryH1H=1Wb/A
Celsius temperatureDegree Celsius1t=T-To
Luminous fluxLumenlm1lm=1cd・sr
IlluminanceLuxlx1lx=1lm/m2

Reference Information

SI unit prefixes indicating integer powers of ten

FactorPrefixSymbolFactorPrefixSymbol
1018exaE10-1decid
1015petaP10-2centic
1012teraT10-3millim
109gigaG10-6microµ
106megaM10-9nanon
103kilok10-12picop
102hectoh10-15femtof
10dekada10-18attoa

Non-SI units

AmountUnit nameUnit symbolDefinition
TimeMinutemin1min=60s
Hourh1h=60min
Dayd1d=24h
Plane angleDegree°1°= (π/180) rad
Minute1′= (1/60) °
Second1″= (1/60) ′
VolumeLiterl, L1l=1dm3
WeightMetric tont1t=103kg