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SI base unit
Links between the seven SI base unit definitions. Clockwise from top: second (time), metre (length), ampere (electric current), mole (amount of substance), kilogram (mass), kelvin (temperature), and candela (luminous intensity).

The International System of Units is the standard modern form of the metric system. The name of this system can be shortened or abbreviated to SI, from the French name Système International d'unités.

The International System of Units is a system of measurement based on 7 base units: the metre (length), kilogram (mass), second (time), ampere (electric current), Kelvin (temperature), mole (quantity), and candela (brightness). These base units can be used in combination with each other. This creates SI derived units, which can be used to describe other quantities, such as volume, energy, pressure, and velocity.

The system is used almost globally. Only Myanmar, Liberia, and the United States do not use SI as their official system of measurement. In these countries, though, SI is commonly used in science and medicine.

History and use

The metric system was created in France after the French Revolution in 1789. The original system only had two standard units, the kilogram and the metre. The metric system became popular amongst scientists.

In the 1860s, James Clerk Maxwell and William Thomson (later known as Lord Kelvin) suggested a system with three base units - length, mass, and time. Other units would be derived from those three base units. Later, this suggestion would be used to create the centimetre-gram-second system of units (CGS), which used the centimetre as the base unit for length, the gram as the base unit for mass, and the second as the base unit for time. It also added the dyne as the base unit for force and the erg as the base unit for energy.

As scientists studied electricity and magnetism, they realized other base units were needed to describe these subjects. By the middle of the 20th century, many different versions of the metric system were being used. This was very confusing.

In 1954, the 9th General Conference on Weights and Measures (CGPM) created the first version of the International System of Units. The six base units that they used were the metre, kilogram, second, ampere, Kelvin, and candela. The seventh base unit, the mole, was added in 1971.

SI is now used almost everywhere in the world, except in the United States, Liberia and Myanmar, where the older imperial units are still widely used. Other countries, most of them historically related to the British Empire, are slowly replacing the old imperial system with the metric system or using both systems at the same time.

Units of measurement

Base units

The SI base units are measurements used by scientists and other people around the world. All the other units can be written by combining these seven base units in different ways. These other units are called "derived units".

SI base units
Unit name Unit
Quantity measured General definition
metre m length
kilogram kg mass
second s time
  • Original (Medieval): 1/86400 of a day
  • Current (1967): The time needed for 9192631770 periods or cycles of the radiation created by electrons moving between two energy levels of the caesium-133 atom.
ampere A electric current
  • Original (1881): A tenth of the abampere, the unit of current used in the electromagnetic CGS.
  • Current (1946): The current passing through two very long and thin wires placed 1 m apart that produces an attractive force equal to 2×10−7 newton per metre of length.
kelvin K temperature
  • Original (1743): The centigrade scale is obtained by assigning 0° to the freezing point of water and 100° to the boiling point of water.
  • Current (1967): The fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
mole mol amount of substance
  • Original (1900): The molecular weight of a substance in mass grams.
  • Current (1967): The same amount as the number of atoms in 0.012 kilogram of carbon-12.
candela cd luminous intensity
  • Original (1946): 1/60 of the brightness per square centimetre of a black body at the temperature where platinum freezes.
  • Current (1979): The 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.

Derived units

Derived units are created by combining the base units. The base units can be divided, multiplied, or raised to powers. Some derived units have special names. Usually these were created to make calculations simpler.

Named units derived from SI base units
Name Symbol Quantity Definition
other units
SI base units
radian rad plane angle
steradian sr solid angle
hertz Hz frequency s−1
newton N force, weight m∙kg∙s−2
pascal Pa pressure, stress N/m2 m−1∙kg∙s−2
joule J energy, work, heat N∙m m2∙kg∙s−2
watt W power, radiant flux J/s m2∙kg∙s−3
coulomb C electric charge s∙A
volt V voltage, electrical potential difference, electromotive force W/A
farad F electrical capacitance C/V m−2∙kg−1∙s4∙A2
ohm Ω electrical resistance, impedance, reactance V/A m2∙kg∙s−3∙A−2
siemens S electrical conductance 1/Ω m−2∙kg−1∙s3∙A2
weber Wb magnetic flux J/A m2∙kg∙s−2∙A−1
tesla T magnetic field strength Wb/m2
henry H inductance Wb/A
degree Celsius °C temperature relative to 273.15 K TK − 273.15 K
lumen lm luminous flux cd∙sr cd
lux lx illuminance lm/m2 m−2∙cd
becquerel Bq radioactivity (decays per unit time) s−1
gray Gy absorbed dose (of ionizing radiation) J/kg m2∙s−2
sievert Sv equivalent dose (of ionizing radiation) J/kg m2∙s−2
katal kat catalytic activity s−1∙mol


Very large or very small measurements can be written using prefixes. Prefixes are added to the beginning of the unit to make a new unit. For example, the prefix kilo- means "1000" times the original unit and the prefix milli- means "0.001" times the original unit. So one kilometre is 1000 metres and one milligram is a 1000th of a gram.

Standard prefixes for the SI units of measure
Multiples Name deca- hecto- kilo- mega- giga- tera- peta- exa- zetta- yotta-
Prefix da h k M G T P E Z Y
Factor 100 101 102 103 106 109 1012 1015 1018 1021 1024
Fractions Name deci- centi- milli- micro- nano- pico- femto- atto- zepto- yocto-
Prefix d c m μ n p f a z y
Factor 100 10−1 10−2 10−3 10−6 10−9 10−12 10−15 10−18 10−21 10−24

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