When we measure the pressure of gases, like that of the air in car tyres, we usually make this measurement relative to normal air pressure. That is, we calculate the difference between the pressure in the tyre and the pressure exerted by our atmosphere. Liquids can also be measured against normal air pressure.
A manometer measures the pressure acting on a column of fluid. It is made from a U-shaped tube of liquid in which the difference in pressure acting on the two straight sections of the tube causes the liquid to reach different heights in the two arms.
The pressure difference can then be calculated by using the following equation:
pressure difference = height × density × g
p = h × p × g