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Units are magnitudes of a physical quantity, defined by convention or law which sets a standard for any measurements of the same physical quantity. For example, mass is a physical quantity, and a kilogram is a unit that measures mass. Therefore any object with mass can be represented as a multiple of one kilogram. All units of a certain physical quantity can be equated to one another, and it is of common practice to use prefixes to do so. These help avoid using large numbers when writing out values, and gives a simple way to communicate them. Since this encyclopedia is focused on energy, the various units representing it will be explored on this page.

There are two primary systems used to define units: the Metric (International) and Imperial systems as seen below. The metric system has a small number (7 specifically) of fundamental units and then multiplies these units together to form derived units.

Unit Converter

This calculator allows for the conversion between different units of a physical quantity including Energy, Mass, Length, and more. For example, it may be useful to know how many feet are in a meter, how many pounds are in a kilogram, or how many joules are in a kilowatt-hour.

International system of units

The international system of units is the modern standardized form of the metric system. It sets standard measurements and conversions, and is the most commonly and universally accepted system of units. The units listed below are the base and derived units for this system.[1]

Base units

Unit name Quantity
Kilogram Mass
Meter Distance
Second Time
Kelvin Temperature
Ampere Electric current
Mole Amount
Candela Luminous intensity

Important Derived units

These units are a combination of base units, used to describe specific physical phenomena like force and voltage.

Unit Name Quantity
Newton (N) Force
Pascal (Pa) Pressure
Joule (J) Energy
Watt (W) Power
Coulomb (C) Charge
Volt (V) Electrical potential difference
Farad (F) Electrical capacitance
Ohm (Ω) Electrical resistance
Celsius (°C) Temperature
Becquerel (Bq) Radioactivity
Sievert (Sv) Radiation dose

Imperial system of units

The imperial system was developed in Britain in the early 19th century, due to the need to establish uniformity among measurements. Many of the units had been described prior to that, however it was formalized by The Weights and Measures Act in 1824.[2] The history of how the units are derived is quite interesting, and can be read about at Dr. Rowlett's Unit Dictionary. Some important units of the system include:

Unit name Quantity
Slug Mass
Foot (ft)
Mile (mi)
Fahrenheit (°F)
Rankine (°R)
Pound (lb) Force
Foot pound Energy
Acre (ac) Area

Energy units

Power units

For Further Reading

For further information please see the related pages below:


  1. BIPM, Measurement units: the SI [Online], Available: http://www.bipm.org/en/measurement-units/
  2. Britannica. (August 25, 2015). British Imperial System [Online], Available: http://www.britannica.com/science/British-Imperial-System