Basic and Derived Units

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 République 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. 

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. 

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

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     Name Symbol 
length Historical Context meter m
mass Historical Context kilogram kg
time Historical Context second s
electric current Historical Context ampere A
thermodynamic temperature Historical Context kelvin K
amount of substance Historical Context mole mol
luminous intensity Historical Context candela cd

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 Quantity Name Expression in terms of SI units
area square meter m2
volume cubic meter m3
speed, velocity meter per second m/s
acceleration meter per second squared m/s2
mass density kilogram per cubic meter kg/m3
force netwon (N) m·kg·s-2
pressure pascal m-1·kg·s-2
energy, work joule (J) N-m m2·kg·s-2
electric potential volt (V) m2·kg·s-3·A-1
current density ampere per square meter A/m-2
electric resistance Ohm m2·kg·s-3·A-2
dynamic viscosity pascal second m-1·kg·s-3
surface tension newton/meter m·kg·s-2/m
magnetic field strength ampere per meter A/m
electric charge Coulumb (C) s·A


More derived units from


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  • Mass vs. Weight - what is the difference between mass and weight?
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  • Density - how to find the density of solids and liquids

  • Pressure- What is pressure, what is atmospheric pressure?
  • Surface Tension- What is surface tension and capillary action?
  • Humidity - How is humidity measured? Absolute, Relative and Specific Humidty, What is dew point?

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  • Work and Energy - Types of Energy - What is Work?
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  • Electric Charge -- What is Electric Charge?
  • Volt - Electromotive Force, Why do electrons flow?
  • Amp -- What is Electric Current?
  • Ohm - Ohms law lab exercise.
  • Magnetism - Properties of Magnets