When a submersible pump dies, the easy move is to drop in the same horsepower that came out of the hole. Sometimes that's right. Often the house has added a bathroom, the water level has changed, or the old pump was the wrong size to begin with. Sizing a pump properly comes down to two numbers: how many gallons a minute the house needs, and how many feet of "head" the pump has to push that water against. This calculator works both out using formulas published by North Dakota State University Extension and residential guidance from the Water Systems Council, then turns them into a rough horsepower range you can take into a conversation with a licensed installer.
The formulas, step by step
- Peak flow in gallons per minute. The Water Systems Council's fixture-count method sets pump capacity equal to the number of water outlets in the home: each sink, tub, shower and toilet, the dishwasher, washer, laundry tub, every hose bib, and special uses like a hot tub. Their own example, a two-bath house with a kitchen sink, dishwasher, washer, laundry tub and two outside spigots, comes to 12 outlets and 12 gpm.
- Lift. Measure from the pumping water level, not the static level. NDSU Extension defines static head for a well as the distance from the water level while pumping up to the ground, plus any climb from there to where the water is delivered.
- Pressure head. A 2.31-foot column of water exerts one pound per square inch, so multiply the switch's shut-off pressure by 2.31. At 60 psi that's about 139 feet, often more than the lift itself on a shallow well.
- Friction. Pipe, fittings and valves eat some head. NDSU says to total these losses from friction tables; your installer knows the pipe size and run length.
- Total dynamic head (TDH) is the sum of lift, rise, pressure head and friction.
- Water horsepower = gpm × TDH ÷ 3,960 (NDSU Extension AE1057).
- Motor horsepower is water horsepower divided by the pump's efficiency, which NDSU says to read off the manufacturer's pump curve.
Where the horsepower range comes from
Efficiency is the one number a homeowner can't look up without picking a specific pump, so the calculator brackets it. The low end assumes about 64 percent: Grundfos's data sheet for its 10S10-15, a 1-horsepower, 4-inch submersible, rates it at 8.81 gpm against 288.7 feet of head, which works out to 0.64 water horsepower from a 1-horsepower motor. The high end assumes 48 percent, the average overall efficiency a 2018 study in the journal Water reported from a survey of installed submersible well pumps. That study was about utility wells and counts motor losses too, so treat it as a cautious ceiling rather than a typical value. Results round up to the motor sizes in Franklin Electric's submersible application manual, which steps from 1/3 to 1/2, 3/4, 1, 1-1/2, 2, 3 and 5 horsepower and beyond.
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A worked example
Take a made-up well with a pumping level of 140 feet, a tank 10 feet uphill in the house, a 40/60 switch, and the Water Systems Council's 12-outlet house: 140 + 10 + (60 × 2.31 = 139) = 289 feet of head before friction. At 12 gpm that's 12 × 289 ÷ 3,960 = 0.88 water horsepower, or roughly 1.4 to 1.8 horsepower at the bracketed efficiencies. In practice that conversation is about a 1-1/2 or 2 horsepower pump, and the installer settles it from the curves.
Finding your own numbers around Oklahoma City
- Depth and water level. Licensed Oklahoma drillers file a completion report for each new well, and OWRB's well record search lets you look them up (OWRB owner's guide; OWRB). The log usually lists total depth and water levels at the time of drilling. Garber-Wellington wells vary a lot from one to the next, so use your own record rather than a neighbor's when you can (USGS).
- Pressure setting. It's printed on or inside the pressure switch cover, usually near the tank. The pressure tank page explains how the switch and tank work together.
- Yield. Also on the well log, or from a test the installer runs. A pump that pulls more than the well makes will draw the water down to the intake.
Why bigger isn't better
The Water Systems Council's guidance is blunt: don't install a pump with more capacity than the well. If the house's peak use beats the well's yield, the fix is to size the pump to the well and add storage, often a larger pressure tank, which also cuts down on starts and stops, the main source of pump wear. That's why the calculator caps flow at your well yield when you enter one. For more on choosing and installing the new unit, see well pump replacement.
Dry taps or a pump that won't quit? One call gets you to a licensed pump installer near you. (405) 536-9841
Sources
NDSU Extension, Irrigation Water Pumps (AE1057), for total dynamic head, 2.31 feet per psi, and the horsepower formulas; the Water Systems Council's article Sizing a Well Pump as published by The Driller, for the fixture-count method, pressure settings and the yield rule; the Grundfos 10S10-15 data sheet; Beck and others, Increasing Energy Efficiency in Water Collection Systems by Submersible PMSM Well Pumps (Water, 2018); and Franklin Electric's AIM manual. All read October 2026. Licensed Oklahoma pump installers are regulated by the Oklahoma Water Resources Board, and every pump installation has to meet the state's minimum standards (OAC 785:35-9-1).
FAQ
How do I know what size well pump I need?
Work out peak flow (about one gallon per minute per water outlet, per the Water Systems Council) and total dynamic head (pumping level plus rise plus pressure setting times 2.31 plus friction). Those two numbers pick the pump from a manufacturer's curve.
Why does pressure count as feet of head?
One psi equals the push of a 2.31-foot column of water, so a pump that has to reach 60 psi at the tank is working as if it lifted water another 139 feet.
Can I put in a bigger pump to get better pressure?
Pressure comes from the switch setting, and flow is limited by the well. A pump with more capacity than the well yields can draw the water down to the intake. Fix low flow with the right pump and enough storage, not extra horsepower.