Density is the quantity of something proportional to the amount of space it takes up. For physical objects, it is an object's mass per unit of volume, usually expressed as grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). To calculate an object's density, divide its mass by its volume.

A luggage of clothes and other travel items has a mass of 18 kilograms, and a volume of 0.1 cubic meters. To calculate the density, do the following:

mass    = 18 kilograms
volume  = 0.1 cubic meters
density = 18 kilograms ÷ 0.1 cubic meters = 180 kg/m3

A luggage that is empty has a mass of only 3.6 kilograms, but the same volume of 0.1 cubic meters. To calculate the density, do the following:

mass    = 3.6 kilograms
volume  = 0.1 cubic meters
density = 3.6 kilograms ÷ 0.1 cubic meters = 36 kg/m3

A luggage that is full of bricks has an enormous mass of 180 kilograms, but the same volume of 0.1 cubic meters. To calculate the density, do the following:

mass    = 180 kilograms
volume  = 0.1 cubic meters
density = 180 kilograms ÷ 0.1 cubic meters = 1,800 kg/m3

Density is the measure of how compact an object is. In the examples above, the luggage of clothes is 5 times more dense than the empty luggage, but 10 times less dense than the luggage of bricks.

Water has a density of 1 g/cm³ or 1,000 kg/m³. An object will float in water if it's less dense than water, and sink otherwise. In the examples above, both the luggage of clothes and empty luggage will easily float in water, but the luggage of bricks will sink.