The definition of density is the mass of a unit volume for a substance. Specific gravity, also known as relative density, is the ratio of the density of a substance to the density of a reference substance. In most cases, the reference substance is pure water at 4 °C (39.2 °F) and 1 atm (760 mmHg, 1.01325 bar, 14.6959 psi).
To obtain the density of a homogeneous solid object, both mass and volume are needed. The mass is normally measured with an appropriate scale or balance; the volume may be measured directly or by the displacement of a fluid. For more general cases, Hydrostatic weighing, Hydrometer, Pycnometer, and Digital density meter are commonly used methods to measure specific gravity.