Water is the traditional liquid used as a reference when comparing density, especially in measurements of relative density or specific gravity. By convention, the density of water is often treated as approximately 1 gram per cubic centimetre, or 1,000 kilograms per cubic metre.
This reference makes comparisons easy. A substance with a specific gravity of 2 is roughly twice as dense as water, while a substance with a value below 1 is less dense and may float. The exact density of water changes with temperature and pressure, so scientific work specifies those conditions.
Water reaches its maximum density at about 3.98°C, where it is approximately 999.972 kilograms per cubic metre at atmospheric pressure. At room temperature, its density is slightly lower, which is why “1 g/cm³” is a useful approximation rather than a universal exact value.
A common mix-up is treating water as the SI standard unit for density itself. Density is measured in units such as kilograms per cubic metre; water is mainly the familiar comparison reference used for relative density and many practical calculations.