In the international system of units (SI), the mass of materials is measured in kilograms and the density in kilograms per cubic metre. In the " CGS " system of physical units, the g/m3, the g or kg/litre are commonly used depending on what is being measured. To quantify the kilogram, the first researchers based it on the litre of water of which the density is 999.95 kg/ m3 (at precisely 4°C).
Relative density is the ratio between the mass of a body and that of the same volume of water. This density is a so-called " derived quantity », as are acceleration, current density, surface area, speed etc. The symbol ρ (Greek letter rho) is an SI convention for density expressed in kg/m3.
A few examples of derived quantities and units.