It is important to precisely evaluate patient dose from a diagnostic X-ray in order to investigate medical exposure reduction. As a method of evaluating patient surface dose, computation with existing data based on exposure in air is generally used. With this method, backscatter factors and absorbed dose conversion factors are given by the parameter of the effective energy or the half value layer, making this procedure complicated. We developed program software (Surface Dose Evaluation Code, SDEC) that computes the surface dose automatically, using the backscatter factor and absorbed dose conversion factor calculated by using X-ray spectral data. Because the measurement of effective energy or a half value layer is unnecessary, SDEC is a useful evaluation method.