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 be 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)^{[clarification needed]} the expression is supposed to have. In this case, the reinsertion of the correct powers of e, c, etc., can be uniquely determined.

Systems of natural units

Stoney units

Stoney system dimensions in SI units

Quantity	Expression	Approx. metric value
Length	${\displaystyle {\sqrt ((Gk_{\text{e))e^{2))/{c^{4))))}$	1.380×10−36 m[1]
Mass	${\sqrt ((k_{\text{e))e^{2))/{G))}$	1.859×10−9 kg[1]
Time	${\displaystyle {\sqrt ((Gk_{\text{e))e^{2))/{c^{6))))}$	4.605×10−45 s[1]
Electric charge	${\displaystyle e}$	1.602×10−19 C

The Stoney unit system uses the following defining constants:

c, G, k_e, e,

where c is the speed of light, G is the gravitational constant, k_e is the Coulomb constant, and e is the elementary charge.

George Johnstone Stoney's unit system preceded that of Planck by 30 years. He presented the idea in a lecture entitled "On the Physical Units of Nature" delivered to the British Association in 1874.^[2] Stoney units did not consider the Planck constant, which was discovered only after Stoney's proposal.

Planck units

Planck dimensions in SI units

Quantity	Expression	Approx. metric value
Length	${\displaystyle {\sqrt ((\hbar G}/{c^{3))))}$	1.616×10−35 m[3]
Mass	${\displaystyle {\sqrt ((\hbar c}/{G))))$	2.176×10−8 kg[4]
Time	${\displaystyle {\sqrt ((\hbar G}/{c^{5))))}$	5.391×10−44 s[5]
Temperature	${\displaystyle {\sqrt ((\hbar c^{5))/{G{k_{\text{B))}^{2))))}$	1.417×1032 K[6]

The Planck unit system uses the following defining constants:

c, ħ, G, k_B,

where c is the speed of light, ħ is the reduced Planck constant, G is the gravitational constant, and k_B is the Boltzmann constant.

Planck units form a system of natural units that is not defined in terms of properties of any prototype, physical object, or even elementary particle. They only refer to the basic structure of the laws of physics: c and G are part of the structure of spacetime in general relativity, and ħ is at the foundation of quantum mechanics. This makes Planck units particularly convenient and common in theories of quantum gravity, including string theory.^{[citation needed]}

Planck considered only the units based on the universal constants G, h, c, and k_B to arrive at natural units for length, time, mass, and temperature, but no electromagnetic units.^[7] The Planck system of units is now understood to use the reduced Planck constant, ħ, in place of the Planck constant, h.^[8]

Schrödinger units

Schrödinger system dimensions in SI units

Quantity	Expression	Approx. metric value
Length	${\displaystyle {\sqrt ((\hbar ^{4}G(4\pi \varepsilon _{0})^{3))/{e^{6))))}$	2.593×10−32 m
Mass	${\displaystyle {\sqrt ((e^{2))/{4\pi \varepsilon _{0}G))))$	1.859×10−9 kg
Time	${\displaystyle {\sqrt ((\hbar ^{6}G(4\pi \varepsilon _{0})^{5))/{e^{10))))}$	1.185×10−38 s
Electric charge	${\displaystyle e}$	1.602×10−19 C[9]

The Schrödinger system of units (named after Austrian physicist Erwin Schrödinger) is seldom mentioned in literature. Its defining constants are:^[10][11]

e, ħ, G, k_e.

Geometrized units

Defining constants:

c, G.

The geometrized unit system,^[12]: 36 used in general relativity, the base physical units are chosen so that the speed of light, c, and the gravitational constant, G, are set to one.

Atomic units

Atomic-unit dimensions in SI units

Quantity	Expression	Metric value
Length	${\displaystyle {(4\pi \epsilon _{0})\hbar ^{2))/{m_{\text{e))e^{2))}$	5.292×10−11 m[13]
Mass	${\displaystyle m_{\text{e))}$	9.109×10−31 kg[14]
Time	${\displaystyle {(4\pi \epsilon _{0})^{2}\hbar ^{3))/{m_{\text{e))e^{4))}$	2.419×10−17 s[15]
Electric charge	${\displaystyle e}$	1.602×10−19 C[16]

The atomic unit system^[17] uses the following defining constants:^{[18]: 349 [19]}

m_e, e, ħ, 4πε₀.

The atomic units were first proposed by Douglas Hartree and are designed to simplify atomic and molecular physics and chemistry, especially the hydrogen atom.^[18]: 349 For example, in atomic units, in the Bohr model of the hydrogen atom an electron in the ground state has orbital radius, orbital velocity and so on with particularly simple numeric values.

Natural units (particle and atomic physics)

Quantity	Expression	Metric value
Length	${\displaystyle {\hbar }/{m_{\text{e))c))$	3.862×10−13 m[20]
Mass	${\displaystyle m_{\text{e))}$	9.109×10−31 kg[21]
Time	${\displaystyle {\hbar }/{m_{\text{e))c^{2))}$	1.288×10−21 s[22]
Electric charge	${\displaystyle {\sqrt {\varepsilon _{0}\hbar c))}$	5.291×10−19 C

This natural unit system, used only in the fields of particle and atomic physics, uses the following defining constants:^[23]: 509

c, m_e, ħ, ε₀,

where c is the speed of light, m_e is the electron mass, ħ is the reduced Planck constant, and ε₀ is the vacuum permittivity.

The vacuum permittivity ε₀ is implicitly used as a nondimensionalization constant, as is evident from the physicists' expression for the fine-structure constant, written α = e²/(4π),^[24][25] which may be compared to the correspoding expression in SI: α = e²/(4πε₀ħc).^[26]: 128

Strong units

Strong-unit dimensions in SI units

Quantity	Expression	Metric value
Length	${\displaystyle {\hbar }/{m_{\text{p))c))$	2.103×10−16 m
Mass	${\displaystyle m_{\text{p))}$	1.673×10−27 kg
Time	${\displaystyle {\hbar }/{m_{\text{p))c^{2))}$	7.015×10−25 s

Defining constants:

c, m_p, ħ.

Here, m_p is the proton rest mass. Strong units are "convenient for work in QCD and nuclear physics, where quantum mechanics and relativity are omnipresent and the proton is an object of central interest".^[27]

In this system of units the speed of light changes in inverse proportion to the fine-structure constant, therefore it has gained some interest recent years in the niche hypothesis of time-variation of fundamental constants.^[28]

Summary table

Quantity	Planck	Stoney	Atomic	Particle and atomic physics	Strong	Schrödinger
Defining constants	${\displaystyle c}$, ${\displaystyle G}$, ${\displaystyle \hbar }$, ${\displaystyle k_{\text{B))}$	${\displaystyle c}$, ${\displaystyle G}$, ${\displaystyle e}$, ${\displaystyle k_{\text{e))}$	${\displaystyle e}$, ${\displaystyle m_{\text{e))}$, ${\displaystyle \hbar }$, ${\displaystyle k_{\text{e))}$	${\displaystyle c}$, ${\displaystyle m_{\text{e))}$, ${\displaystyle \hbar }$, ${\displaystyle \varepsilon _{0))$	${\displaystyle c}$, ${\displaystyle m_{\text{p))}$, ${\displaystyle \hbar }$	${\displaystyle \hbar }$, ${\displaystyle G}$, ${\displaystyle e}$, ${\displaystyle k_{\text{e))}$
Speed of light ${\displaystyle c}$	${\displaystyle 1}$	${\displaystyle 1}$	${\displaystyle {1}/{\alpha ))$	${\displaystyle 1}$	${\displaystyle 1}$	${\displaystyle {1}/{\alpha ))$
Reduced Planck constant ${\displaystyle \hbar }$	${\displaystyle 1}$	${\displaystyle {1}/{\alpha ))$	${\displaystyle 1}$	${\displaystyle 1}$	${\displaystyle 1}$	${\displaystyle 1}$
Elementary charge ${\displaystyle e}$	—	${\displaystyle 1}$	${\displaystyle 1}$	${\displaystyle {\sqrt {4\pi \alpha ))}$	—	${\displaystyle 1}$
Vacuum permittivity ${\displaystyle \varepsilon _{0))$	—	${\displaystyle {1}/{4\pi ))$	${\displaystyle {1}/{4\pi ))$	${\displaystyle 1}$	—	${\displaystyle {1}/{4\pi ))$
Gravitational constant ${\displaystyle G}$	${\displaystyle 1}$	${\displaystyle 1}$	${\displaystyle {\eta _{\mathrm {e} ))/{\alpha ))$	${\displaystyle \eta _{\mathrm {e} ))$	${\displaystyle \eta _{\mathrm {p} ))$	${\displaystyle 1}$

where:

• α is the fine-structure constant (α = e² / 4πε₀ħc ≈ 0.007297)
• η_e = Gm_e² / ħc ≈ 1.7518×10⁻⁴⁵
• η_p = Gm_p² / ħc ≈ 5.9061×10⁻³⁹
• A dash (—) indicates where the system is not sufficient to express the quantity.