1. Definition

The density (or relative density) of a homogeneous solid or liquid is the ratio of its density to the density of a reference body. The conditions (pressure, temperature, etc.) may be different for the two bodies. The density of the reference body (usually water) refers to the ratio of the density of a water sample at temperature θ (°C) to its maximum value, which is reached at a temperature very close to 4°C, with the water free of dissolved gases and under a pressure of 101,325 Pa (1 atm). Temperatures refer to the 1968 International Temperature Scale (see article Thermometric scales [R 2 510] in the Measurement and Control treatise).

The SI international system unit for density is the kilogram per cubic meter (kg · m ^–3 ).