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-The Metric System of Measurements

The Metric System of Measurements or more properly the International System of Units (in French the Système international d'unités or SI) was formally established in 1960 by the General Conference on Weights and Measures (CGPM), but its origins date back to the sixteenth century.  The CGPM represents 52 member states and 26 further associate members.  At the time of establishment, the SI system had 6 base units - the meter, kilogram, second, ampere, degree kelvin (subsequently renamed the "kelvin") and candela and 22 derived units. More units have subsequently been added.

For more information see the NIST Guide for the use of the International System of Units.

Base Units

Meter (m) for length: The length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.

Kilogram (kg) for mass: Equal to the mass of the international prototype of the kilogram.

Second (s) for time: The duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Ampere (A) for electric current: A constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.

Kelvin (K) for thermodynamic temperature: The fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Candela (cd) for luminous intensity: 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.

Mole (mol)
for quantity of matter: The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.


Non-SI Units Commonly Used

Ångström (Å) for measure of displacement: Equal to 0.0000000001 meter (10-10m). Used to express wavelengths of visible light, UV light, gamma rays, and x-rays.

Liter (L) for volume: Equal to 1000 cm3.


SI Derived Units

Derived Quantity Name
Expression in Terms of
Other SI Units
Expression in Terms of
SI Base Units
plane angle radian rad - m·m-1=1 
solid angle steradian sr - m2·m-2=1
frequency hertz Hz - s-1
force newton N - m·kg·s-2
pressure, stress pascal Pa N/m2 m-1·kg·s-2
energy, work, quantity of heat joule J Nm m2·kg·s-2
power, radiant flux watt W J/s m2·kg·s-3
electric charge, quantity of electricity coulomb C - s·A
electric potential, difference, electromotive force volt V W/A m2·kg·s-3·A-1
capacitance farad F C/V m-2·kg-1·s4·A2
electric conductance siemens A/V m-2·kg-1·s3·A2
electric resistance ohm W V/A m2·kg·s-3·A-2
magnetic flux weber Wb Vs m2·kg·s-2·A-1
magnetic flux density tesla T Wb/m2 kg·s-2·A-1
inductance henry H Wb/A m2·kg·s-2·A-2
Celsius temperature degree Celsius °C - K
luminous flux lumen lm cd·sr m2·m-2·cd = cd
illuminance lux lx lm/m2 m2·m-4·cd = m-2·cd
activity (of a radionuclide) becquerel Bq - s-1
absorbed dose, specific energy (imparted), kerma gray Gy J/kg m2·s-2
dose equivalent sievert Sv J/kg m2·s-2
catalytic activity katal kat - s-1mol


SI Prefixes
Prefix   Symbol Factor
Yocto Septillion y 10-24  
Zepto Sextillion z 10-21  
Atto Quintillionth a 10-18  
Femto Quadrillionth 10-15  
Pico Trillionth 10-12  
Nano Billionth 10-9  
Micro  Millionth µ  10-6   
Milli Thousandth 10-3   
Centi Hundredth c 10-2   
Deci  Tenth 10-1   
Base Unit One   100
Deca  Ten da  101 
Hecto Hundred 102 
Kilo Thousand 103 
1 000
Mega Million 106 
100 0000 
Giga Billion 109 
1000 000 000 
Tera Trillion 1012
1000 000 000 000 
Peta Quadrillion 1015 
1 000 000 000 000 000
Exa Quintillion 1018 
1 000 000 000 000 000 000 
Zetta Sextillionth 1021 
1 000 000 000 000 000 000 000
Yotta Septillionth 1024
1 000 000 000 000 000 000 000 000 


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