Head measures pressure in a fluid, but uses units of length instead of units of force per area. Piezometric head combines pressure head -- the head due to pressure at a certain point in the fluid -- with elevation head, the head due to the distance between that point and a datum level. The piezometric head stays constant at every point if a body of liquid is at rest, but changes by location if the liquid is moving. Choose the point at which you want to calculate piezometric head before beginning the calculation.
Multiply the pressure at the point you want to calculate piezometric head by 144 to convert from pounds per square inch (psi) to pounds per square foot (psf). If the pressure is 20 psi, for example, (20 psi)*(144 psf/psi) = 2,880 psf.
Divide the pressure by the specific weight of the substance to find the pressure head. The unit weight of water is 62.4 lb/ft^3. Use a table to look up the unit weights of other substances, some which can be found at The Engineering ToolBox website. For water at a pressure of 2,880 psf, the pressure head is (2,880 psf)/(62.4 lb/ft^3) = 46.2 ft.
Find the distance from your datum to the point at which you are calculating piezometric pressure to determine the elevation head. Depending on your location, the datum could be the bottom of an aquifer, the lowest point of a piping system or just an arbitrarily chosen location. For example, if you are performing the calculation for an aquifer and using the bottom as the datum, a location 5 feet above the datum would have an elevation head of 5 feet.
Add the pressure head to the elevation head to find the piezometric head. For example, 46.2 ft + 5 ft = 51.2 feet of piezometric head.