Classical electron radius

The classical electron radius, also known as the Lorentz radius or the Thomson scattering length, is based on a classical (i.e. non-quantum) relativistic model of the electron. According to modern understanding, the electron is a point particle with a point charge and no spatial extent. Attempts to model the electron as a non-point particle are considered ill-conceived and counter-pedagogic. However, the classical electron radius is calculated as (in SI units)

where ${\displaystyle e}$ and ${\displaystyle m_{\text{e}}}$ are the electric charge and the mass of the electron, ${\displaystyle c}$ is the speed of light, and ${\displaystyle \varepsilon _{0}}$ is the permittivity of free space.

...
Wikipedia