A modification of a previously described Fourier-transform-based algorithm for calculating isotope distributions is presented. It allows one to calculate ultrahigh resolution isotope distributions on computers having limited memory resources and incudes a peak-shape function as an inherent part of the calculation. The method zooms-in on a limited mass range and uses digital filtering to reduce the required array size for the calculation. Alternative methods typically have very unfavorable scaling properties as molecular weight increases, making them unsuitable for this application. At present, the method is probably the only practicable way to calculate ultrahigh resolution isotope distributions of very large molecules using computers having limited memory resources.