A standard operating procedure (SOP) has been developed for measuring particle size distribution of gas atomised steel powder in Mastersizer 2000 from Malvern Instrument Ltd. The measuring principle is based on laser diffraction. This SOP is optimised for gas atomised steel powders used for manufacturing of powder material by hot isostatic pressing (HIP). Using the SOP, consistent results are achieved. In order to measure representative samples and to minimise the standard deviation between successive measurements the sampling method is important. Rotary sample splitting is recommended, as it gives better consistency between samples compared to spoon sampling.