Water pressure, sometimes referred to as hydrostatic pressure, deals with fluids that are static. While movement above the water surface from waves and ripples can make calculating water pressure on the surface difficult, water pressure at lower water depths is easier to calculate. Pipe sizing standards may differ around the world, but pipe sizes can be determined using the thickness of the pipe walls and circumference of the pipe.

Determine the depth, in meters, of the object for which you wish to calculate the water pressure.

Measure the atmospheric pressure, in kilograms per meter-second squared (kg/ms²), of the water surface using a barometer.

Observe the standard values of water density and gravitational acceleration of earth. The standard value for water density is 1000kg/m³ and the value for earth's gravitational acceleration is 9.81m/s².

Plug the values you found in Steps 1 to 3 into this equation to find the water pressure: P = A + (L x G) where "P" represents the water pressure, "A" represents the atmospheric pressure at the water's surface, "L" represents water density and "G" represents the gravitational acceleration.

To express the pressure in lb./inch^2 (pound per square inch), multiply the answer for "P" by .014.

Wrap a thin piece of twine around the pipe to measure the circumference of the pipe. Mark the length where the two ends of the string meet and measure the distance with a ruler.

Divide the circumference measured in Step 1 by the mathematical value of pi, which is 3.14. The answer given represents the outer diameter (OD) of the pipe.

Measure the width of the pipe walls. With the edge of the exposed pipe and a ruler, measure from the outside edge of the pipe to the inner wall lining.

Compare your measurements with a standardise pipe chart (see Resources). Using the measurement of the OD and width of the pipe walls, find the row and column associated with the pipe size.