# Natural units

In physics, **natural units** are physical units of measurement in which only universal physical constants are used as defining constants, such that each of these constants acts as a coherent unit of a quantity. For example, the elementary charge *e* may be used as a natural unit of electric charge, and the speed of light *c* may be used as a natural unit of speed. A purely natural system of units has all of its units defined such that each of these can be expressed as a product of powers of defining physical constants.

Through nondimensionalization, physical quantities may then redefined so that the defining constants can be omitted from mathematical expressions of physical laws, and while this has the apparent advantage of simplicity, it may entail a loss of clarity due to the loss of information for dimensional analysis. It precludes the interpretation of an expression in terms of constants, such as *e* and *c*, unless it is *known* which units (in dimensionful units) the expression is supposed to have. In this case, the reinsertion of the correct powers of *e*, *c*, etc., can be uniquely determined.