Extrusion–spheronisation is a widely used technique for the manufacture of pellets with high sphericity and narrow size distribution. A dimensionless framework for describing the evolution of pellet shape with spheronisation time is presented for the first time and is validated using new experimental data obtained with two families of materials: (i) microcrystalline cellulose (MCC)/water-based pastes with loadings of up to 15 wt.% calcium carbonate representing a ‘hard’ active pharmaceutical ingredient, and (ii) a lactose/MCC/water paste. The dimensional analysis of the pellet rounding stage identified the paste density and bulk yield strength, σY, as scaling quantities: σY was measured in separate extrusion tests and found to increase with increasing carbonate content. Larger paste strength gave longer spheronisation times and less spherical pellets for a given set of spheronisation conditions. The pellet aspect ratio was found to increase in a linear manner with the logarithm of spheronisation time, progressing towards an asymptotic final value. This behaviour, which is evident in old data sets but has not been discussed previously, is compared with two simple models. High speed imaging was also used to examine the collision behaviour of pellets during the breakage and rounding stages in spheronisation. This confirmed that the rounding phase was the rate-determining step. The velocities of a number of tracked pellets were consistently lower than the tip speed of the rotating friction plate, confirming previous findings in studies of beds of pellets.