Specific gravity is defined as the ratio of fluid density to the density of a reference substance, both defined at the same pressure and temperature. These densities are usually defined at standard conditions (14.7 psia and 60°F). For a condensate, oil or a liquid, the reference substance is water:
The value of water density at standard conditions is 62.4 lbm/ft3 approximately. For a natural gas, or any other gas for this matter, the reference substance is air:
Or, equivalently, substituting Equation (18.2) evaluated at standard conditions ( for most gases),
where the value of the molecular weight for air is MWair = 28.96 lbm/lbmol. Specific gravity is nondimensional because both numerator and denominator have the same units.