Every pump has a head curve which shows head vs GPM. To determine GPM, you need to know the plumbing head loss and to find that you can measure the pump pressure on the return side and the pump suction. Most pumps have drain plugs which work fairly well to measure both pressure and suction with a pressure and suction gauge. Once both measurements are done, you can find head loss with the following equations:

Return Head (ft) = Pump Pressure (PSI) * 2.31

Suction Head (ft) = Pump Suction (in-hg.) * 1.13

Total Head (ft) = Return Head + Suction Head

Once you have Total Head, you can look up the GPM on the pump's head curve.


There are a couple of other methods that are a little easier to do but slightly less accurate. For example, you can use the filter pressure gauge to determine return head but then you need to use a slightly different equation to calculate the return head because the gauge is above the pump:

Return Head (ft) = Filter Pressure (PSI) * 2.31 + 3


I can also estimate the flow rate by knowing your pump model, filter pressure and a description of the suction side plumbing (pipe diameter, length and number of runs).