Quantitative theory for the lateral momentum distribution after strong-field ionization

► We investigate the electron momentum distribution after strong-field ionization of atoms. ► A quantitative tunneling formula is derived for the lateral width of the distribution. ► The pre-exponential factor differs from previous tunneling formulas. ► We compare to numerical results for hydrogen and to previous experiments for Ar and Ne. ► Quantitative agreement is found when the correct prefactor is taken into account.