SI Units

The 11th General Conference on Weights and Measures (1960) adopted the name Système International d'Unités (International System of Units, international abbreviation SI), for the recommended practical system of units of measurement. There are seven base units which by convention are regarded as dimensionally independent. Derived units are those formed by combining base units according to the algebraic relations linking the corresponding quantities.



SI Base Units


1 m / metre | Unit of Length

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.


1 kg / kilogram | Unit of Mass

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.


1 s / second | Unit of Time

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 caesium 133 atom.


1 A / ampere | Unit of Electric Current

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 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.


1 K / Kelvin | Unit of Thermodynamic Temperature

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


1 mol / mole | Unit of amount of Substance

1. 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.
2. 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 particles.


1 cd / candela | Unit of Luminous Intensity

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.


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SI Derived Units


Coherent derived units in the SI with special names and symbols:


1 rad / radian | Unit of plane angle; Expressed in terms of SI base units: 1 rad = 1 m/m ;

1 sr / steradian | Unit of solid angle; Expressed in terms of SI base units: 1 sr = 1 m^2/m^2 ;

1 Hz / hertz | Unit of frequency; Expressed in terms of SI base units: 1 Hz = 1/s ;

1 N / newton | Unit of force; Expressed in terms of SI base units: 1 N = 1 m*kg / s^2 ;

1 Pa / pascal | Unit of pressure, stress; Expressed in terms of SI base units: 1 Pa = 1 kg / m*s^s ;

1 J / joule | Unit of energy, work, amount of heat; Expressed in terms of SI base units: 1 J = 1 m^2 * kg / s^2 ;

1 W / watt | Unit of power, radiant flux; Expressed in terms of SI base units: 1 W = 1 m^2 * kg / s^3 ;

1 C / coulomb | Unit of electric charge, amount of electricity; Expressed in terms of SI base units: 1 C = 1 s*A ;

1 V / volt | Unit of electric potential difference, electromotive force; Expressed in terms of SI base units: 1 V = 1 m^2 * kg / (s^3 * A) ;

1 F / farad | Unit of capacitance; Expressed in terms of SI base units: 1 F = 1 s^4 * A^2 / (m^2 * kg) ;

1 / ohm | Unit of electric resistance; Expressed in terms of SI base units: 1 ohm = 1 m^2 * kg / (s^3 * A^2) ;

1 S / siemens | Unit of electric conductance; Expressed in terms of SI base units: 1 S = 1 s^3 * A^2 / (m^2 * kg) ;

1 Wb / weber | Unit of magnetic flux; Expressed in terms of SI base units: 1 Wb = 1 m^2 * kg / (s^2 * A) ;

1 T / tesla | Unit of magnetic flux density; Expressed in terms of SI base units: 1 T = 1 kg / (s^2 * A) ;

1 H / henry | Unit of inductance; Expressed in terms of SI base units: 1 H = 1 m^2 * kg / (s^2 * A^2) ;

1 °C / degree Celsius | Unit of Celsius temperature; Expressed in terms of SI base units: 1 °C = 1 K ;

1 lm / lumen | Unit of luminous flux; Expressed in terms of SI base units: 1 lm = 1 cd ;

1 lx / lux | Unit of illuminance; Expressed in terms of SI base units: 1 lx = 1 cd / m^2 ;

1 Bq / becquerel | Unit of activity referred to a radionuclide; Expressed in terms of SI base units: 1 Bq = 1 / s ;

1 Gy / gray | Unit of absorbed dose, specific energy (imparted), kerma; Expressed in terms of SI base units: 1 Gy = 1 m^2 / s^2 ;

1 Sv / sievert Unit of dose equivalent, ambient dose equivalent, directional dose equivalent, personal dose equivalent; Expressed in terms of SI base units: 1 Sv = 1 m^2 / s^2 ;

1 kat / katal | Unit of catalytic activity; Expressed in terms of SI base units: 1 kat = 1 mol / s ;







Coherent derived units in the SI expressed in terms of base units:



Derived quantity: Area A; SI coherent derived unit: square metre m^2 ;

Derived quantity: Volume V; SI coherent derived unit: cubic metre m^3 ;

Derived quantity: Speed, velocity; SI coherent derived unit: metre per second m / s ;

Derived quantity: Acceleration a; SI coherent derived unit: metre per second squared m / s^2 ;

Derived quantity: Wavenumber k; SI coherent derived unit: reciprocal metre 1 / m ;

Derived quantity: Density, mass density; SI coherent derived unit: kilogram per cubic metre kg / m^3 ;

Derived quantity: Surface density A; SI coherent derived unit: kilogram per square metre kg / m^2 ;

Derived quantity: Specific volume v; SI coherent derived unit: cubic metre per kilogram m^3 / kg ;

Derived quantity: Current density j; SI coherent derived unit: ampere per square metre A / m^2 ;

Derived quantity: Magnetic field strength H; SI coherent derived unit: ampere per metre A / m ;

Derived quantity: Amount concentration, concentration c; SI coherent derived unit: mole per cubic metre mol / m^3 ;

Derived quantity: Mass concentration , SI coherent derived unit: kilogram per cubic metre kg / m^3 ;

Derived quantity: Luminance Lv; SI coherent derived unit: candela per square metre cd / m^2 ;

Derived quantity: Refractive index n; SI coherent derived unit: one 1 ;

Derived quantity: Relative permeability r; SI coherent derived unit: one 1 ;






SI Prefixes:


10^–24 yocto y

10^–21 zepto z

10^–18 atto a

10^–15 femto f

10^–12 pico p

10^–9 nano n

10^–6 micro µ

10^–3 milli m

10^–2 centi c

10^–1 deci d

10^1 deca da

10^2 hecto h

10^3 kilo k

10^6 mega M

10^9 giga G

10^12 tera T

10^15 peta P

10^18 exa E

10^21 zetta Z

10^24 yotta Y