Private wells · flow demand
Well pump GPM needed calculator.
Count fixtures, or add up the flow of fixtures running at the same time, to estimate the pump capacity a household needs, and see how much storage a low-yield well would call for. Runs in your browser; nothing is sent anywhere.
Planning aid: codes and soils vary by location. Confirm septic and well work with your local health department and a licensed professional.
Inputs
Results
Worked example
The Water Systems Council fixture-count method for a house with 12 fixtures and outlets gives 12 GPM. For peak use, two showers at 2.5 GPM, a washer at 3 GPM and a sink at 1.5 GPM running together total 9.5 GPM. The higher figure, 12 GPM, is 4 GPM above a well that sustains only 8 GPM, so covering a 20 minute peak needs 4 × 20 = 80 gallons from storage.
Formulas, sources, and limits
Formulas: Fixture count: required GPM = number of fixtures and outlets. Peak demand: sum the flow rates of the fixtures that run together at the busiest time. Shortfall = max(0, required GPM − sustained well yield). Storage = shortfall × peak minutes. The storage line is simple arithmetic added by this tool; the source only states that storage must be added when the well yields less than the peak requirement.
Sources (retrieved 2026-10): Water Systems Council, Sizing a Well Pump (The Driller, Nov. 2006); Pelikan, Tech Topics: Sizing a Residential Water System, Part 1: Flow (The Driller, June 2007).
Limits: The fixture-count rule is a rule of thumb. The source example puts peak flow at roughly 12 to 20 GPM for two showers, a washing machine and a sink depending on fixture age; your fixture flows may differ, and irrigation can set the real peak. The tool does not choose a pump; use the pump curve against TDH.
Frequently asked questions
How many GPM does a house well need?
The Water Systems Council rule of thumb is one GPM per fixture or outlet, so a house with 12 fixtures needs about 12 GPM. Peak simultaneous use is the other method.
What if my well produces less than the peak demand?
The source states that storage capacity must be added when the well’s sustained yield is below the peak requirement.
Should I add every fixture’s maximum flow?
No. Fixtures rarely all run at once, so use the fixtures likely to run together at the busiest time.
Is the pressure tank related to GPM?
Yes; the pressure tank size is set from pump GPM and run time. See the well pressure tank sizing calculator.