A change of source skin distance results in a significant change in percentage depth dose. The factorsP(f2) / P(f1)by which percentage depth dose at SSDf1is to be multiplied to find new percentage depth dose at SSDf2are tabulated and presented in a graphical form, forf1 = 100 cmandf2 = 110–140 cmfor 8 MV x rays and forf1 = 80 cmandf2 = 90–140 cmfor60Co gamma rays. The use of quadratic and linear equations to calculate this factor have been suggested. The ratios calculated using a quadratic equation agree very well with values from experimental data.