Abstract This study examines the distribution of measured diameters of single particles. Diameter distributions of six particles were compared with the theoretical gamma, chisquared, and Student's t distributions. Five aluminum oxide particles of varying shape, and a nitrocellulose spheroid were each repeatedly sized thirty-two times using a light-blockage particle counter. For each observation, an equivalent diameter was calculated from the projected cross-sectional area of the particle. Mean diameters and standard deviations were then calculated from these values. A qualitative description of each particle's shape was determined using a scanning electron microscope. The shapes ranged from spheroidal to flake-like. Using the Student t distribution and specified confidence levels, an estimate was also made of the minimum number of trials necessary to reproduce the experimental results.