The formula for density, which is mass divided by volume, can be manipulated to have the volume as the unknown. Given the mass and the density, the volume can be found by dividing the mass by the density.
The International System of Units (SI) unit used for density is kilogram per cubic meter. The densities of common substances are known constants. Thus, looking for either the mass or the volume is easy by simply using the formula above. Density can be observed when a helium balloon escapes and floats through the air. This happens because the air around the balloon is more dense than helium.