Natural units

In physics, natural unit systems are measurement systems for which certain physical constants have been set to 1 through nondimensionalization of physical units. For example, the speed of light $c$ may be set to 1, and it may then be omitted, equating mass and energy directly $E = m$ rather than using $c$ as a conversion factor in the typical mass–energy equivalence equation $E = mc 2$ . A purely natural system of units has all of its dimensions collapsed, such that the physical constants completely define the system of units and the relevant physical laws contain no conversion constants.

While natural unit systems simplify the form of each equation, it is still necessary to keep track of the non-collapsed dimensions of each quantity or expression in order to reinsert physical constants (such dimensions uniquely determine the full formula). Dimensional analysis in the collapsed system is uninformative as most quantities have the same dimensions.