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Specific Gravity is defined as the ratio of the density of a given substance to the density of water, when both are at the same temperature, it is therefore a dimensionless quantity (see below). Substances with a specific gravity greater than one are denser than water (and so will sink in it), and those with a specific gravity of less than one are less dense than water (and so will float in it). Specific gravity is a special case of, or in some usages synonymous with, relative density, with the latter term often preferred in modern scientific writing.

Specific gravity, SG, is expressed mathematically as:

where is the density of the substance, and is the density of water. (By convention ρ, the Greek letter rho, denotes density.) The density of water varies with temperature and pressure, and it is usual to refer specific gravity to the density at 4°C (39.2°F) and a normal pressure of 1 atm. In this case is equal to 1000 kg·m⁻³ in SI units (or 62.43 lb·ft⁻³ in United States customary units).

Given the specific gravity of a substance, its actual density can be calculated by inverting the above formula:

Occasionally a reference substance other than water is specified (for example, air), in which case specific gravity means density relative to that reference.

Specific gravity is by definition dimensionless and therefore not dependent on the system of units used (e.g. slugs·ft⁻³ or kg·m⁻³). However, the two densities must of course be converted to the same units before carrying out the numerical ratio calculation.