Several methods (condensation approximation, regularization, and non-negative least-squares (NNLS)) have been applied for calculating the pore size distribution (PSD) function from simulated isotherms. It has been shown that two near by peaks in the low size region may be recovered well by both regularization and NNLS method only for isotherms without experimental error. Wide peaks may be recovered by numerical methods almost independently on a number of experimental points and error inherent in input data. Condensation approximation is the most insensitive to the experimental errors. However it gives a distorted one peak distribution shifted to low values of micropore size and having a long tail at large values. In the case of isotherms with large errors (10% for regularization and 5% for NNLS), both numerical methods give PSD functions close to that obtained by the CB method.