Differential pressure is a measurement used in liquid systems to calculate the differences in pressure at different points within the system. For example, a plumber may need to calculate the difference in pressure levels at different points along the route that water travels within a home's pipes. A number of factors affect the final value for the differential pressure. It's a more complicated calculation than simply measuring the pressure of the system at two points and subtracting.

Determine the change in pressure between the two points in the system. You will need an instrument such as a pressure gauge to determine the pressure at two points. Subtract the smaller number from the larger. Call this value "P".

Measure the diameter of the pipe that the liquid travels through. Call this value "d".

Determine the density of the fluid that you are measuring. Call this value "p".

Determine the rate of flow of the liquid, also known as the mass flow rate. Call this value "W".

Calculate the differential pressure using the following equation: K = (1.59923 x P x (d^4) x p)/(W^2)

Calculate "d" to the fourth power and multiply the answer by P x p x 1.59923.

Calculate the square of "W".

Divide the answer in step 6 by the answer in step 7 to determine the differential pressure.