In an investigation of the penetration of a beam of fast electrons in a solid, a simple empirical method has been used for calculating the distribution of the energy dissipation by electrons in a semi-infinite target. The calculations are based on the values of the distribution function tabulated by Spencer for an infinite target. The method has been applied to obtain the distribution for 25, 50 and 100 keV electrons at normal incidence in aluminium, copper, tin and lead. The calculated differences in the dissipation distribution between the infinite and the semi-infinite cases agree fairly well with the results of the Monte Carlo calculations by Bishop. A comparison with the experimental distribution measured in a metal-oxide-semiconductor system is also presented.