Well pump GPM = one gallon per minute per fixture, and horsepower = GPM × total dynamic head ÷ 3,960 ÷ efficiency. A house with 12 fixtures needs 12 GPM; with water 150 ft down, 10 ft of drawdown, 10 ft of pipe friction and 50 psi at the tank (115.5 ft), the total head is 285.5 ft and the pump needs 1.44 hp, so a 1 1/2 hp submersible.
How the well pump size is calculated
Sizing a well pump takes two answers: how much water the house asks for at peak, and how hard the pump has to push it.
GPM = number of fixtures that can run at once × 1 GPM
total dynamic head = depth to water + drawdown + lift to the tank + friction + psi × 2.31 ft
motor hp = GPM × total dynamic head ÷ (3,960 × pump and motor efficiency)
The 1 GPM per fixture rule is the standard peak-demand method: The Driller counts a house with two full bathrooms (sink, tub or shower, toilet), a kitchen sink, a basement sink, two outside faucets, a washing machine and a dishwasher as 12 GPM. This calculator counts each bathroom as 3 fixtures. One psi of pressure equals 2.31 ft of head, so a 50 psi system adds 115.5 ft. The 3,960 constant converts gallons per minute times feet of head into horsepower.
Worked example: two bathrooms, water at 150 ft
| Step | Calculation | Result |
|---|---|---|
| Fixtures | 2 baths × 3 + kitchen + dishwasher + washer + laundry tub + 2 hose bibs | 12 GPM |
| Pressure as head | 50 psi × 2.31 | 115.5 ft |
| Total dynamic head | 150 + 10 + 0 + 10 + 115.5 | 285.5 ft |
| Hydraulic power | 12 × 285.5 ÷ 3,960 | 0.87 hp |
| Motor power at 60% | 0.87 ÷ 0.6 | 1.44 hp |
| Motor size | next size up | 1 1/2 hp |
The same house with one bathroom, no laundry tub and one hose bib is 7 GPM and 0.84 hp of motor power, a 1 hp pump; with three bathrooms it is 15 GPM and 1.80 hp, a 2 hp pump.
Well pump size chart
Motor size by depth to water and flow at 10 ft of drawdown, 10 ft of friction, 50 psi and 60% efficiency:
| Depth to water | 8 GPM | 10 GPM | 12 GPM | 15 GPM |
|---|---|---|---|---|
| 50 ft | 3/4 hp | 1 hp | 1 hp | 1 1/2 hp |
| 100 ft | 1 hp | 1 hp | 1 1/2 hp | 1 1/2 hp |
| 200 ft | 1 1/2 hp | 1 1/2 hp | 2 hp | 3 hp |
| 300 ft | 1 1/2 hp | 2 hp | 3 hp | 3 hp |
Change any input and the chart on the calculator recalculates. The efficiency is an input: 60% is a planning value, and the pump curve for the model you buy gives the real figure. Well Pump Repair Authority notes a 1 hp submersible in a 200 ft well produces roughly 10 to 15 GPM, which is in line with the chart.
What kind of well pump for the depth
InspectAPedia's guide by depth to water: up to 25 ft a one-line (shallow well) jet pump; 25 to 90 ft a two-line (deep well) jet pump; 90 to 250 ft a submersible; over 250 ft a multi-stage submersible. The calculator names the type for your depth. Jet pump depth limits vary by model, so confirm with the pump maker.
Do not out-pump the well
The pump must not draw more than the well yields. Your well driller's report gives the yield in GPM. If the house asks for 12 GPM and the well gives 5, add a storage tank or size the pump to the yield: The Driller's advice is to never install a pump with greater capacity than the well. Oakville Pump makes a related point: an oversized pump runs off its efficient zone and can pump the well down. Enter the yield in the calculator and it warns you.
Friction and pressure
Friction in the drop pipe is the most overlooked part of the head. The calculator takes it as a number in feet: 10 ft suits a short run of drop pipe sized to the pump discharge, and long or narrow pipe adds more. Use your switch's cut-out pressure, 60 psi for a 40/60 switch, when you want the pump to hold the top of its range. The water pressure calculator converts head to psi and back.
The next questions
- Treating the water? Size a water softener from the same household.
- Basement water problems? The sump pump size calculator sizes the other pump in the house.
- Wiring the pump? Use the wire size calculator.
- Draining a run downhill? The pipe slope calculator gives the fall.
- See every heating, cooling and plumbing calculator.