This paper presents a simple and effective method for calculating the pore size distribution (PSD) of a porous solid from the adsorption isotherm of a probe adsorbate. In this method, the pore size spectrum is divided into a number of subranges, each of which is represented by a mean pore size. Provided a set of single pore isotherms for those mean pore sizes are available, the optimization procedure is then invoked to find the volume of each subrange such that the calculated isotherm matches the experimental data. Result of the optimization is a histogram of the PSD. The method is simple and always converges to the same solution irrespective of the starting initial condition and the number of subranges.