Density of biodiesel is an important physical property of liquid fuel and biodiesel. A slight change in density can affect engine output power. In this work, density of a saturated, unsaturated fatty acid methyl ester or a biodiesel can be estimated from either (1) its number of carbon atoms (of fatty acid, z) and number of double bonds (n(a)): ln rho = -0.43S - 0.0025z + 85.98/T + 0.792z/T + 4.0 n(a)/T or (2) its saponification number (SN) and iodine value (IV): in rho = -0.427 10/SN + 83.38/T + 3168.95/(T x SN) + 11 IV (T x SN), where T is absolute temperature. The predicted densities at different temperatures from both equations agree well with the reported literature values.