Basic, derived SI units, prefixes and constants
Basic, derived SI units, prefixes and constants

Basic, derived SI units, prefixes and constants

In Europe, the units for measuring different properties are known as SI units. SI stands for the International System of Units (Système International d’Unités, abbreviated SI). It comprises a coherent system of units of measurement starting with seven base units. The SI unit system is informally known as the metric system. Scroll down and find out all about basic, derived SI units, prefixes and constants.
All units are derived from the seventh base units.

SI units

Base quantityBase unitSymbol
Electrical currentAmpereA
Luminous intensityCandelacd
Amount of substanceMolemol
SI base units

The SI electrical units are based on the mksa (meter-kilogram-second-ampere) system. They have been adopted by the standardization bodies of the world, including the International Electrotechnical Commission (IEC), the American National Standards Institute (ANSI), and the Standards Board of the Institute of Electrical and Electronics Engineers (IEEE). The United States is the only industrialized nation in the world that does not mandate the use of the SI system.

Although the U.S. Congress has the constitutional right to establish measuring units, it has never enforced any system. The metric system (now SI) was legalized by Congress in 1866 and is the only legal measuring system, but other non-SI units are legal as well.

The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units.

Derived SI units

Derived quantityNameSymbol
Energy, work, quantity of heatJouleJ
Electric charge, quantity of electricityCoulombC
Potential difference, electromotive forceVoltV or U
Electrical resistanceOhmΩ
Magnetic fluxWeberWb
Luminous fluxLumencd
AreaSquare metrem2
VolumeCubic metre
Velocity, speedMetre per secondm/s
Mass densityKilogram per cubic metrekg/m³
LuminanceCandela per square metrecd/m2
SI electrical derived units

SI unit prefixes

Like all metric systems, the SI uses metric prefixes to systematically construct, for one and the same physical quantity, a whole set of units of widely different sizes that are decimal multiples of each other.

The current version of the SI provides twenty metric prefixes that signify decimal powers ranging from 10−24 to 1024. Apart from the prefixes for 1/100, 1/10, 10, and 100, all the other ones are powers of 1000.

SI unit prefixes


mA | Milliamp = one thousandth of an ampere
km | Kilometre = one thousand metres
µv | Microvolt = one millionth of a volt
GW | Gigawatt = one thousand million watts
kW | Kilowatt = one thousand watts

Calculator example:

1 kilometre is 1 metre × 103
Enter into calculator 1 EXP 3 = (1000) metres
1000 metres is 1 kilometre × 10-3
Enter into calculator 1000 EXP −3 = (1) kilometre
1 micro volt is 1 volt × 10-6
Enter into calculator 1 EXP −6 = (1−06 or 0.000 001) volts
(note sixth decimal place).

SI Defining Constants

The SI rests on a foundation of seven values, known as the constants. The values of the constants are the same everywhere in the universe. In the SI, these constants completely define the seven base SI units:

SymbolNameExact value
{\displaystyle \Delta \nu _{\text{Cs}}}
hyperfine transition frequency of Cs

9192631770 Hz
cspeed of light299792458 m/s
hPlanck constant6.62607015×10−34 J⋅s
eelementary charge1.602176634×10−19 C
kBoltzmann constant1.380649×10−23 J/K
NAAvogadro constant6.02214076×1023 mol−1
Kcdluminous efficacy of 540 THz radiation683 lm/W
SI constants

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This Post Has One Comment

  1. Johan Bisal


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