Examinations of past complex theoretical equations describing isothermal dielectric constants (_[epsilon]_) of liquids show that none describe experiments throughout and that only a two-constant equation of density is required. The dielectric susceptibility (_[epsilon]_ – 1) as a simple exponential function of density is shown here to describe _[epsilon]_ for both polar and non-polar organic liquids over all available experimental ranges from 0.50 to 1.2 g cm^-3^ and temperatures to 400^o^C. For water above 0.25 g cm^-3^, the equation describes the excellent dielectric-constant formulation of Fernández, et al., (1997) throughout its experimental range [-35 to +600^o^C; to 1000 MPa (10000 Atm)] and even when diluted by "inert" solvents (Marshall, 2008). At lower densities at all temperatures, water susceptibilities converge to an even more simple unit proportionality to density below 0.003 g cm^-3^, not yet verified for organics without experiment. While the past Born, Tait, Clausius-Mossotti, and Owen-Brinkley theories describe _[epsilon]_ equally well over a limited 10% change in density, all are shown to fail over greater ranges. The simple density relation presented here should be useful in clarifying and reducing complexity of theory applied to liquid structure.