Density is the ratio of the mass of a substance or object to its volume. Substances that are light for their size have a low density, and vice versa. Objects whose density is less than that of water will float in water, while a hot air balloon will rise when its average density becomes less than that of air. In the metric system, density is measured in grams per cubic centimeter (or kilograms per liter). The term is also applied to properties other than mass; e.g., charge density refers to the ratio of electric charge to volume.

Densities of familiar substances and objects
object / substance density (g/cm3)
water 1.0
iron 7.89
Sun 1.41
white dwarf 2 × 106
neutron star 7 × 1014
Venus 5.2
Earth 5.52
Mars 3.94
Jupiter 1.31
Saturn 0.69
Uranus 1.29
Neptune 1.64
Pluto 2.03