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'''Natural units''' are units of [[measurement]] that arise when a physical [[constant]], such as [[Planck's constant]] or the [[speed of light]] in a vacuum, is set to a [[dimensionless]] value such as one.<ref>[http://superstringtheory.com/unitsa.html Natural units]</ref>

'''Natural units''' are units of [[measurement]] that arise when a physical [[constant]], such as [[Planck's constant]] or the [[speed of light]] in a vacuum, is set to a [[dimensionless]] value such as one.<ref>[http://superstringtheory.com/unitsa.html Natural units]</ref> For example, in natural units, since ''c'' is set to unity, mass and energy can be expressed in the same units.  Therefore, the [[mass (science)|masses]] of [[subatomic particle]]s are often expressed in in [[Electron-Volts|electron volts]], which are a unit of [[energy]].

[[Radian]]s used in [[geometry]] are one example of a natural constant, since they are defined so that [[pi]] radians make a half-turn (more widely known as 180 degrees).

[[Radian]]s used in [[geometry]] are one example of a natural constant, since they are defined so that [[pi]] radians make a half-turn (more widely known as 180 degrees).