The formula "mass = density x volume" is a variation on the density formula: density = mass ÷ volume. As long as two of the variables are known, the third can be calculated by rearranging the equation. In order to calculate mass, it is necessary to divide both sides of the density equation by volume.

For example, to find the mass of a liter of milk, it is first necessary to know the density of the milk. If the density is 1.03 g/mL, then according to the equation, the mass of one liter of milk is 1.030 kilograms.

Mass is often confused with weight, although they are two different things. Mass is an unchanging quality of an object. Relativistic mass, however, can increase when an object approaches light speed. It is a measurement of the amount of matter the object has. When a physicist discusses the mass of an object, he or she is is talking about the number of particles in the object, not how much they weigh. Weight, on the other hand, is the force of gravity pulling on a mass. The SI unit of mass, as determined by the International System of Units, is the kilogram.

Density is how much mass there is in a given amount of a material. An object of higher density will weigh more than an object of lower density, even if the objects are the same size.