Specific Gravity Calculator

Specific gravity (also called relative density) is a dimensionless ratio of the density of a material to the density of a reference substance — typically water at 4°C (39.2°F) for liquids and solids, or dry air at 20°C for gases. Because it's a ratio, specific gravity has no units.

For liquids and solids, divide the material's density by the density of water at 4°C (1000 kg/m³ or 1 g/cm³): SG = ρ_material / ρ_water. For gases, you can either divide the gas density by the density of air, or divide the molecular weight of the gas by the molecular weight of dry air (28.965 g/mol).

Yes, specific gravity and relative density are effectively the same concept. Both describe the ratio of a substance's density to a reference substance's density. 'Specific gravity' is the more common term in everyday and industrial use, while 'relative density' is preferred in formal scientific contexts.

Water is used as the reference because it is universally available, well-studied, and has a conveniently round density of 1000 kg/m³ at 4°C. This makes calculations intuitive — a specific gravity greater than 1 means the material is denser than water and will sink, while less than 1 means it will float.

For gases, specific gravity is typically defined as the ratio of the molecular weight of the gas to the molecular weight of dry air (28.965 g/mol). Alternatively, it can be the ratio of the gas density to air density at the same temperature and pressure. Dry air at 20°C and 1 atm has a density of approximately 1.205 kg/m³.

A specific gravity less than 1 for a liquid or solid means the material is less dense than water and will float on water. For example, most woods, ice, and oils have a specific gravity below 1. A value greater than 1 means the material is denser than water and will sink.

Specific gravity is dimensionless — it has no units. Because it is a ratio of two densities, the units cancel out regardless of whether you use kg/m³, g/cm³, lb/ft³, or any other density unit, as long as both densities are expressed in the same unit.

Mercury has a specific gravity of about 13.6, meaning it is 13.6 times denser than water. Iron is approximately 7.87, aluminum is about 2.7, ice is 0.917, and ethanol is around 0.789. Water itself has a specific gravity of exactly 1.0 at 4°C.