Temperature and pressure vary from place to place, with time of day and season. It is necessary to define an easy to remember standardised temperature and pressure so that physicists can agree on the results of experiments. We take the standard temperature to be 0 ° C - the freezing point of water at a pressure of 101.3125 kPa. This is then defined as the standard pressure.

Another commonly used temperature and pressure are 'Normal Temperature and Pressure', NTP. The normal temperature is defined to be 20 ° and the normal pressure is defined to be 101.3125 kPa.

The reason for using 101.3125 kPa as a pressure is that it is equal to the pressure exerted by a column of 760 mm of Mercury, or the average atmospheric pressure at sea level.