Well pump sizing calculator
Count your fixtures or pick your bedrooms and bathrooms for the flow, then enter the pumping water level, the pressure and the pipe run. You get the total dynamic head and the duty point to look up on a pump curve, plus the pressure tank drawdown that goes with it.
Inputs
Result
Friction loss in well pipe
Feet of head lost per 100 ft of polyethylene or Schedule 40 PVC pipe (Hazen-Williams, C = 150). Your flow is marked.
| gpm | 1 in | 1-1/4 in | 1-1/2 in | 2 in |
|---|---|---|---|---|
| 5 | 1.5 | 0.4 | 0.2 | 0.1 |
| 8 | 3.6 | 1.0 | 0.5 | 0.1 |
| 10 | 5.5 | 1.4 | 0.7 | 0.2 |
| 12 | 7.7 | 2.0 | 1.0 | 0.3 |
| 15 | 11.6 | 3.1 | 1.4 | 0.4 |
| 20 | 19.8 | 5.2 | 2.5 | 0.7 |
| 25 | 30.0 | 7.9 | 3.7 | 1.1 |
How a well pump is sized
Two numbers pick a well pump: the flow the house needs at its busiest, in gallons per minute, and the total dynamic head it has to push that flow against, in feet. Together they are the duty point. Find it on the pump maker’s curve; the right pump delivers at least that flow at that head, and the curve also tells you the motor size.
Step 1: the flow
The Water Systems Council’s fixture-count method allows 1 gpm for every fixture: count each faucet, toilet, shower, tub, dishwasher, washing machine, outdoor hose bib and treatment unit. A two-bath house with a kitchen, a washer and a softener counts about 10 fixtures, so 10 gpm. If you water outdoors with more than that, size for the irrigation instead.
The BC plumbing apprenticeship textbook gives a peak demand table by bedrooms and bathrooms:
| Bedrooms | 1 bath | 1-1/2 bath | 2 to 2-1/2 bath | 3 to 4 bath |
|---|---|---|---|---|
| 2 | 6 gpm | 8 gpm | 10 gpm | — |
| 3 | 8 gpm | 10 gpm | 12 gpm | — |
| 4 | 10 gpm | 12 gpm | 14 gpm | 16 gpm |
| 5 | — | 13 gpm | 15 gpm | 17 gpm |
| 6 | — | — | 16 gpm | 18 gpm |
Step 2: the total dynamic head
The pumping water level is how deep the water sits while the pump runs, which is lower than the static level you see at rest; the difference is the drawdown in the well report. One psi is 2.31 ft of water, so a 60 psi cut-out adds 139 ft. Friction comes from the pipe size, length and flow.
Worked example
Three bedrooms and two baths: 12 gpm. Water at 150 ft while pumping, the tank 10 ft above the wellhead, a 40/60 switch, and 220 ft of 1-1/4 in pipe with 25% for fittings:
So the pump has to deliver 12 gpm at about 304 ft. That is 0.92 horsepower delivered to the water (12 x 304 ÷ 3,960); the motor is larger, by the pump’s efficiency, and the curve shows which.
When the well can’t keep up
A pump should never pull more than the well yields over time, or it will draw the water down to the intake. If the house needs more than the well gives, the usual answer is a storage tank filled slowly by the well pump, with a booster pump feeding the house.
The pressure tank goes with it
The pump should run at least a minute each time it starts, so the pressure tank needs at least one gallon of drawdown for each gpm of pump flow: 12 gallons for a 12 gpm pump. The pressure tank sizing calculator turns that into a tank size for your switch setting. Supply pipe inside the house is sized with the water pipe sizing calculator.
Questions
How do I calculate what size well pump I need?
Work out the flow, about 1 gpm per fixture or from a bedrooms-and-baths table, and the total dynamic head: pumping water level, plus the rise to the tank, plus the pressure in psi times 2.31, plus pipe friction. Then find a pump whose curve delivers that flow at that head.
What size pump do I need for a 300 foot well?
It depends on where the water sits, not the depth of the hole. If it draws down to 250 ft while pumping, with a 40/60 switch and 330 ft of 1-1/4 in pipe, a 10 gpm pump has to work against about 395 ft of head. Look for a 10 gpm pump that delivers at that head on its curve.
How many gpm does a house need from a well?
About 1 gpm per fixture by the Water Systems Council method: a typical two-bath house is around 10 gpm. The BC apprenticeship table gives 12 gpm for three bedrooms and two baths, and 14 gpm for four bedrooms and two baths.
What is total dynamic head on a well pump?
The total height and resistance the pump works against, in feet of water: the depth to the pumping water level, the rise to the pressure tank, the pressure converted to feet (psi x 2.31), and the friction in the pipe and fittings.
What pressure should I use for a well pump calculation?
The pressure switch cut-out, such as 60 psi on a 40/60 switch. The pump has to reach that pressure to shut off; at the cut-in pressure it delivers more flow.
What if my well yield is lower than the gpm I need?
Don't pump more than the well gives. The usual fix is a storage tank filled slowly by the well pump and a booster pump to supply the house.
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