Hi, Assuming the electron is a sphere of finite extent, the classical electron radius is derived by equating the energy of the electric field with mec2. For the computation of the field energy, we have to assume a charge distribution. Both constant charge density and constant surface density lead to prefactors (3/5 or 1/2) which are somehow just ignored. Why? If we are only interested in the order of magnitude, why drop these prefactors but keep 1/4?