In physics, the Stoney units form a system of units named after the Irish physicist George Johnstone Stoney, who first proposed them in 1874 (but published only in 1881). They are the earliest example of natural units, i.e., a coherent set of units of measurement designed so that chosen physical constants fully define and are included in the set.

The constants that Stoney used to define his set of units is the following:

Later authors typically replace the Coulomb constant with ⁠1/4πε₀⁠.

This means that the numerical values of all these constants, when expressed in coherent Stoney units, is equal to one:

$${\displaystyle {\begin{aligned}c&=1\ l_{\text{S}}\cdot t_{\text{S}}^{-1}\\G&=1\ l_{\text{S}}^{3}\cdot t_{\text{S}}^{-2}\cdot m_{\text{S}}^{-1}\\k_{\text{e}}&=1\ l_{\text{S}}^{3}\cdot t_{\text{S}}^{-2}\cdot m_{\text{S}}\cdot q_{\text{S}}^{-2}\\e&=1\ q_{\text{S}}\end{aligned}}}$$

In Stoney units, the numerical value of the reduced Planck constant is

$${\displaystyle \hbar ={\frac {1}{\alpha }}~l_{\text{S}}^{2}\cdot t_{\text{S}}^{-1}\cdot m_{\text{S}}\approx 137.036~l_{\text{S}}^{2}\cdot t_{\text{S}}^{-1}\cdot m_{\text{S}}^{~},}$$

where α is the fine-structure constant.