Quantitative studies of salt structures are based on the assumption of a simple relationship between excess sediment volumes and the volumetric growth of salt structures. This assumption is inadequate and, instead, this article shows how the excess volume relates to the volume swept out by the deforming sediment surface and the moving salt/sediment interface. The use of this new basis for volume reckoning is demonstrated, and its implications for waste disposal, hydrocarbon exploration and tectonic subsidence studies are emphasized.