The density and refractive index of 15 room temperature ionic liquids (ILs) grouped according to whether they had the same cation or anion have been measured at atmospheric pressure in a temperature range from 293.15 K to 343.15 K. Changes in the physical properties have been quantified as a function of temperature and alkyl chain length for each cation or anion family. It has been observed that ionic liquid density increases when the alkyl length of the cation decreases but not with the molecular weight of the anion. We have proposed a new method to assign the ionic volume of each component in an IL that explains the trend, followed by all experimental data in this work, and allows us to predict the molecular volume and density of many ILs prior to synthesis. It was also seen that the refractive index increases as alkyl chain length of the cation increases. The temperature-dependent correlation between density and refraction index for the ILs studied has been quantified.