Density

When we talk about density, we're actually talking about mass density, the mass per unit volume:
ρ =
m
V

If an object has a lower density than a surrounding fluid, the object floats in the fluid; if the object has a higher density it sinks.

A density is sometimes specified as a specific gravity, which is the ratio of its density to the density of water at 4°C. For instance, aluminum has a specific gravity of 2.7, which means that its 2.7 times more dense than water. The specific gravity of gold is 19.3.

Densities

Material (or object) Density (kg/m3)
Interstellar space 10-20
Air (20°C and 1 atmosphere) 1.21
Water (4°C and 1 atmosphere) 1000
Iron7900
Mercury (the metal)13600
Earth (the planet, on average)5500
Sun (on average) 1400
Black hole 1019