Beta decay can be considered as a perturbation as described in quantum mechanics, and thus Fermi's Golden Rule can be applied. This leads to an expression for the kinetic energy spectrum  of emitted betas as follows:
where  is the kinetic energy,  is a shape function that depends on the forbiddenness of the decay (it is constant for allowed decays),  is the Fermi Function (see below) with Z the charge of the final-state nucleus,  is the total energy,  is the momentum, and  is the Q value of the decay. The kinetic energy of the emitted neutrino is given approximately by  minus the kinetic energy of the beta.
As an example, the beta decay spectrum of Bi (originally called RaE) is shown to the right.