There are two formulas that may be used to calculate mass: mass is equal to the volume of an object multiplied by its density (m=v*d) and mass is equivalent to an object's weight divided by the acceleration of gravity (m=w/g). The appropriate formula depends on the available variables.
If the density of an object is 11 and its volume is 12, 11 is multiplied by 12 to give a mass of 132 units. The formula can be rearranged to solve for another component. The volume of an item is equivalent to its mass divided by its density (v=m/d). If the mass is 20 and the density is 2, 20 divided by 2 gives a volume of 10.
When an object's weight is 30 and the acceleration of gravity is 3, 30 divided by 3 gives a mass of 10.