Density

Density is the ratio of mass to volume.

D = m/V

Variable Quantity Units
D density \cfrac{\si{g}}{\si{mL}}
m mass \si{g}
V volume \si{mL}

Water’s density is 1 g/mL.
Density less than 1 g/mL: floats in water
Density more than 1 g/mL: sinks in water

“Dense” is not the same as “heavy.” People get these mixed up, but the following examples might help.

Example 1:
A tiny speck of gold has a very low mass (perhaps 0.001 g), but because it’s gold it still has a very high density (19 g/mL).

Example 2:
The earth’s atmosphere has a huge total mass (5 quadrillion tons), but its density is very low (0.001 g/mL at sea level) because it’s made of air.