Density is a measure of mass per unit of volume. Density is a measure of mass per volume. The average density of an object equals its total mass divided by its total volume. An object made from a comparatively dense material (such as iron) will have less volume than an object of equal mass made from some less dense substance (such as water). Perhaps the highest density known is reached in neutron star matter. The singularity at the centre of a black hole, according to general relativity, does not have any volume, so its density is undefined.