There are certain times when you’ll need to calculate the flow rate in Gallons per Minute (GPM) from your well pump. It's actually a fairly easy process and calculation. First, if you have a filter or Mazzei with bypass as I have here, before the pressure tank, you will need to remove the filter or put the Mazzei fully on bypass before proceeding.
1) Run water either at a faucet or by running water through your water treatment equipment until the pump turns on. You will hear a click from the control to tell you when the pump has turned on.
2) Shut off the flow of water and time how long it takes (in seconds) for the pump to turn off. Write down this time.
3) Get something to measure an mount of water, like this 5 gallon jug, and run water from the hose bib at the base of the pressure tank until the pump comes on again, Write down the number of gallons.
4) Calculate the flow rate by inserting the info into this formula
Flow Rate (Gallons Per Minute) GPM = # of gallons / seconds/60
For example if you measured 6 gallons and it took 45 seconds it would be:
Flow Rate = 6 divided by 45/60 or 6 / .75 = 8 gpm flow rate.
5) If you removed a filter or put a Mazzei injector on bypass do not forget to put it back on line.