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.
Things You'll Need
 Pressure gauge
 Calculator

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.
Tips & Warnings
 Differential pressure gauges are available. These gauges monitor the pressure at two points and automatically calculate and update the differential pressure before displaying it on a single dial, making them much easier and more practical to use than doing the calculation manually.
References
 Photo Credit pressure image by Pavel Vlasov from Fotolia.com