Free calculator
Well pump GPM calculator
Gallons from your source meter, divided by run time from your pump hour meter, gives the pump's average rate in gallons per minute. It's the simplest early warning that a well or pump is changing.
Showing an example: one well over eight days. Take both pairs of readings at the same moment, and at least a few days apart. Hour meters that count in tenths make short intervals imprecise.
30.0 gallons per minuteSee results
How it's worked out: 49,390 gal ÷ (27.4 h × 60 min) = 30.0 GPM. That's the pump's average rate while it ran, not its peak.
What the number tells you
Take the calculation every month or so with readings a week or more apart, and keep the results. A single GPM figure means little on its own; a steady drift does. A pumping rate that falls over months can point to a worn pump, a dropping water level in the well, or scale and iron building up in the well screen or pump intake. Bring that trend to your well contractor.
If the rate holds steady but run hours climb, the pump isn't weaker. It's being asked for more water. That's higher demand or a leak, and the meter usage calculator and the water loss calculator will show which.
Run hours per day and time running
Add dates to your readings and the calculator also shows run hours per day and the share of time the pump ran. A pump that runs most of the day has little left for peak demand: summer, a full park, a fire. Hour meters count time, not starts, so rapid short cycling (a pressure tank or switch problem) won't show here. Watch for it at the pressure gauge.
Getting a clean measurement
- Read the meter and the hour meter at the same moment, and write both down together.
- Use readings at least several days apart. A tenth of an hour is 6 minutes, so short intervals are rough.
- If the meter counts cubic feet, the calculator converts at 7.48 gallons per cubic foot. Apply any register multiplier (×10, ×100) before entering. See how to read a water meter.
- A meter reset or replacement between the two readings makes the result meaningless. Start fresh after it.
Quick conversions
- Gallons per hour = GPM × 60.
- Gallons per day at continuous pumping = GPM × 1,440. Actual daily production depends on run hours.
Questions
What's a normal GPM for a well pump?
There isn't one number. It depends on the pump, the well and the pressure the system runs at. Compare against what the pump delivered when it was installed (the well or pump test record, if you have it) and against your own history. The trend matters more than the figure.
Why is my calculated GPM lower than the pump's rating?
A pump's rated flow applies at one specific total head. In service, flow depends on the pumping water level and the pressure the pump works against, so real-world GPM is usually lower than the headline rating. The pump's performance curve shows the whole range.
My pump doesn't have an hour meter. Can I still find the GPM?
Yes, for a quick check. Read the source meter when the pump starts, time the run with a watch, and read the meter again when it stops. Gallons ÷ minutes is the rate for that run. For ongoing tracking, an hour meter wired to the pump circuit is inexpensive; have an electrician install it.
Two wells feed the same meter. Does this work?
Only if a single hour meter counts the time either pump runs, and even then you get a blended rate. For a per-well figure, each well needs its own meter and hour meter, or run one well at a time while you measure.