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Units and Measurement

International System of Units

Background Information

"The creation of the decimal Metric System at the time of the French Revolution and the subsequent deposition of two platinum standards representing the meter and the kilogram, on 22 June 1799, in the Archives de la Rpublique in Paris can be seen as the first step in the development of the present International System of Units.. read on...


A quantity in the general sense is a property ascribed to phenomena, bodies, or substances that can be quantified for, or assigned to, a particular phenomenon, body, or substance. Examples are mass and electric charge.

A quantity in the particular sense is a quantifiable or assignable property ascribed to a particular phenomenon, body, or substance. Examples are the mass of the moon and the electric charge of the proton.

A physical quantity is a quantity that can be used in the mathematical equations of science and technology.

A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value.

All physical quantities can be expressed in terms of seven base units.

Base Quantity   NameSymbol
lengthHistorical Contextmeterm
massHistorical Contextkilogramkg
timeHistorical Contextseconds
electric currentHistorical ContextampereA
temperatureHistorical ContextkelvinK
amount of substanceHistorical Contextmolemol
luminous intensityHistorical Contextcandelacd

Derived Units

Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units. Examples of such SI derived units are given in Table 2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted.


Derived QuantityNameExpression in terms of SI units
areasquare meterm2
volumecubic meterm3
speed, velocitymeter per secondm/s
accelerationmeter per second squaredm/s2
mass densitykilogram per cubic meterkg/m3
forcenetwon (N)mkgs-2
energy, workjoule (J) N-mm2kgs-2
electric potentialvolt (V)m2kgs-3A-1

More derived units.

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