Volume parameters for room-temperature ionic liquids (RTILs) and salts were developed. For 59 of the most common imidazolium, pyridinium, pyrrolidinium, tetralkylammonium, and phosphonium-based RTILs, the mean absolute deviation (MAD) of the densities is 0.007 g cm-3; for 35 imidazolium-based room-temperature salts, the MAD is 0.020 g cm-3; and for 150 energetic salts, the MAD is 0.035 g cm-3. The experimental density (Y) for an alkylated imidazolium or pyridinium-based room-temperature ionic liquid is approximately proportional to its calculated density (X) in the solid state: Y = 0.948X - 0.110 (correlation coefficient: R2 = 0.998, for BF4-, PF6-, NTf2- -containing ionic liquids); Y = 0.934X - 0.070 (correlation coefficient: R2 = 0.999, for OTf-, CF3CO2-, N(CN)2- -containing ionic liquids).