In SI units, number density is measured in m, although cm is often used. However, these units are not quite practical when dealing with atoms or molecules of gases, liquids or solids at room temperature and atmospheric pressure, because the resulting numbers are extremely large (on the order of 10). Using the number density of an ideal gas at  and  as a yardstick:  is often introduced as a unit of number density, for any substances at any conditions (not necessarily limited to an ideal gas at  and ).