Many laboratory rodents grind their food into crumbs that are discarded at the bottom of the cage (sometimes called orts). This can have substantial impacts on measures of food intake and assimilation efficiency. We quantified food grinding in two laboratory mouse strains on eight different diets and distinguished between two hypotheses of why food grinding occurs: a stereotypic behaviour due to a lack of environmental enrichment, or part of an optimal food intake strategy. Orts were quantified when mice were exposed to environmental enrichment and when offered diets of differing energetic quality. Grinding was significantly different between diets, but not between strains, although there was a significant diet by strain interaction. Ort production was lowest on the hardest diets. Not accounting for orts could affect food intake estimates by up to 31.8% and assimilation efficiency by up to 16.7%. Environmental enrichment increased physical activity, but did not reduce grinding. Mice selected the higher energy density components of the food. We suggest a refinement of the current methodology for measuring food intake is essential, primarily because failure to take ort production into account created inaccurate estimates of food intake and assimilation efficiency in mice. Adding environmental enrichment is unlikely to reduce food grinding, but careful choice of diet will reduce the errors.