Saturation vapor density (SVD) is a concept closely tied with saturation vapor pressure. It is useful for getting an exact quantity of water vapor in the air from a relative humidity (RH). Given an RH percentage, the density of water in the air is given by RH * SVD = Actual Vapor Density. Alternatively, RH can be found by RH = Actual Vapor Density / SVD. As Relative Humidity is a percent, vapor density can be in whatever units are necessary, usually grams or kilograms per cubic meter.
For low temperatures (below 400 kelvins or so), SVD can be approximated from the SVP by the ideal gas law: PV = nRT where n is the number of moles, which is related by density by n = M/m, where M is the mass of water present and m is the molar mass of water (18.01528 grams/mole). Thus, setting V to 1 cubic meter, we get P*m/(R*T) = M/V = density.
R is the gas constant, and T is the temperature in kelvin.