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 research, the electron is assumed to be a point particle with a point charge and no spatial extent. However, the classical electron radius is calculated as

$r_{\mathrm {e} }={\frac {1}{4\pi \varepsilon _{0}}}{\frac {e^{2}}{m_{\mathrm {e} }c^{2}}}=2.8179403267(27)\times 10^{-15}\mathrm {m}$ In cgs units, this becomes more simply

$r_{\mathrm {e} }={\frac {e^{2}}{m_{e}c^{2}}}=2.8179403267(27)\times 10^{-13}\mathrm {cm}$ with (to three significant digits)

$e=4.80\times 10^{-10}\mathrm {esu} ,m_{e}=9.11\times 10^{-28}\mathrm {g} ,c=3.00\times 10^{10}\mathrm {cm/s} \,$ .

Using classical electrostatics, the energy required to assemble a sphere of constant charge density, of radius $r_{e}$ and charge $e$ is

$E={\frac {3}{5}}\,\,{\frac {1}{4\pi \varepsilon _{0}}}{\frac {e^{2}}{r_{\mathrm {e} }}}$ .

If the charge is on the surface the energy is

$E={\frac {1}{2}}\,\,{\frac {1}{4\pi \varepsilon _{0}}}{\frac {e^{2}}{r_{\mathrm {e} }}}$ .

Ignoring the factors 3/5 or 1/2, if this is equated to the relativistic energy of the electron ($E=mc^{2}$ ) and solved for $r_{e}$ , the above result is obtained.

In simple terms, the classical electron radius is roughly the size the electron would need to have for its mass to be completely due to its electrostatic potential energy - not taking quantum mechanics into account. We now know that quantum mechanics, indeed quantum field theory, is needed to understand the behavior of electrons at such short distance scales, thus the classical electron radius is no longer regarded as the actual size of an electron. Still, the classical electron radius is used in modern classical-limit theories involving the electron, such as non-relativistic Thomson scattering and the relativistic Klein–Nishina formula. Also, the classical electron radius is roughly the length scale at which renormalization becomes important in quantum electrodynamics.

$r_{e}={\alpha \lambda _{e} \over 2\pi }=\alpha ^{2}a_{0}$ Extrapolating from the initial equation, any mass $m_{0}$ can be imagined to have an 'electromagnetic radius' similar to the electron's classical radius.

$r={\frac {k_{C}e^{2}}{m_{0}c^{2}}}={\frac {\alpha \hbar }{m_{0}c}}$ 