What size well pump do I need?
Sized from total dynamic head, and capped at the well's sustainable yield rather than the house's peak demand.
The short answer
Size a well pump on two numbers: the flow you need in gallons per minute, and total dynamic head — the pumping water level, plus the rise to your tank, plus cut-out pressure at 2.31 feet per psi, plus friction in the pipe. Most homes need 6 to 12 GPM. Never size above what the well sustains.
Total dynamic head
300ft
- Vertical liftPumping level plus the rise above the wellhead.
- 160 ft
- Pressure headCut-out pressure x 2.31 ft per psi.
- 139 ft
- Friction loss0.92 ft per 100 ft at this flow and pipe size.
- 1.6 ft
- Water horsepowerThe useful work. Before any losses.
- 0.45 HP
- Shaft horsepower neededAssuming 50% wire-to-water efficiency, which is typical but varies by model.
- 0.91 HP
- Nearest standard motor
- 1 HP
What to do with these numbers
Pump curves are published per model. Find 6 GPM along the bottom of the curve, read up to 300 ft of head, and any pump whose curve passes above that point will do the duty. Prefer one where that point sits near the middle of the curve rather than at either end — pumps run most efficiently and last longest there.
The horsepower above is a starting point for reading that curve, not a substitute for it. Efficiency varies enough between models that two 1 HP pumps can do noticeably different duties.
Total dynamic head is the whole calculation
A pump does not care how deep your well is. It cares how much work it has to do, and that work is measured as total dynamic head — the equivalent height of water the pump is effectively lifting. Three things add up to it.
- Vertical lift. From the water surface while the pump is running up to the pressure tank. This is the number most people get wrong: the pumping level is deeper than the static level, because drawing water down lowers it. Sizing off the static level under-sizes the pump.
- Pressure head. The pump has to make cut-out pressure, not average pressure. One psi is 2.31 feet of head, so a 40/60 switch adds about 139 feet before the pump has lifted anything at all.
- Friction loss. Water rubbing against pipe walls. It rises steeply with flow and falls steeply with diameter, which is why pipe size matters more than people expect.
Add those three and you have the head. Take that and your GPM to a manufacturer’s pump curve — that is what actually selects a pump. Horsepower is a consequence of the duty, not an input to it.
The rule almost every guide leaves out
Published sizing advice tells you to work out the household’s peak demand and buy a pump that meets it. That is only half a method, and following it is how wells get ruined.
A pump cannot produce water the well does not have. If the house wants 12 GPM and the well sustains 5, a 12 GPM pump draws the water level down to the intake, starts pulling air, loses its prime and overheats — the motor is cooled by the water flowing past it, so running dry destroys it quickly.
The correct answer when demand exceeds yield is not more horsepower. It is a pump matched to the yield, filling a storage tank steadily through the day, with a booster pump delivering household pressure from that tank. It costs less than the pump you would otherwise burn out, and it is the only arrangement that works on a low-yield well.
Typical head and motor size by depth
Worked through the calculator above at 10 GPM, a 40/60 pressure switch, 1.25 inch plastic drop pipe and a 10 foot rise to the tank. Your own numbers will differ — this is for orientation, not selection.
| Pumping water level | Total dynamic head | Nearest standard motor |
|---|---|---|
| 100 ft | 252 ft | 1.5 HP |
| 150 ft | 303 ft | 2 HP |
| 200 ft | 355 ft | 2 HP |
| 300 ft | 457 ft | 3 HP |
| 400 ft | 559 ft | 3 HP |
| 500 ft | 662 ft | 5 HP |
Motor sizes assume 50% wire-to-water efficiency, which is typical for a 4 inch residential submersible but varies by model. Two pumps of the same horsepower can do measurably different duties — always confirm against the curve.
Why drop pipe size changes the pump you need
Friction loss scales with flow to roughly the power of 1.85 and falls with diameter to the power of 4.87. In practice that means moving up one pipe size roughly cuts friction to a third.
| Flow | 1" pipe | 1.25" pipe | 1.5" pipe |
|---|---|---|---|
| 5 GPM | 1.9 ft | 0.7 ft | 0.3 ft |
| 10 GPM | 7.0 ft | 2.4 ft | 1.0 ft |
| 15 GPM | 14.8 ft | 5.0 ft | 2.1 ft |
| 20 GPM | 25.2 ft | 8.5 ft | 3.5 ft |
Head loss in feet per 100 feet of plastic pipe, by the Hazen-Williams equation. On a 400 foot well at 15 GPM, the difference between 1 inch and 1.25 inch pipe is around 40 feet of head — often a whole motor size, paid for every month in electricity rather than once in pipe.
Frequently asked questions
- What size well pump do I need for my house?
- Most homes need 6 to 12 GPM, estimated at roughly one gallon per minute per fixture that might run at once. But flow is only half the answer — the pump also has to overcome total dynamic head, and it must not exceed what your well sustainably yields.
- How deep should a well pump be set?
- Deep enough to stay well below the pumping water level, and typically 10 to 20 feet above the bottom of the well so it is not drawing sediment. The driller's log gives both numbers. Setting a pump too low pulls grit that wears the impellers; too high and it loses prime as the level drops.
- Does a well pump need a pressure tank?
- Yes. Without one the pump would start and stop every time a tap opened, and starting is what wears a pump out. The tank stores pressurised water so the pump runs in longer, less frequent cycles.
- How long should a well pump cycle?
- At least one minute of running per start for motors under 1 HP, and two minutes for 1 HP and above. Shorter than that is short-cycling, and it is the fastest way to destroy a pump. The fix is almost always a bigger or properly pre-charged pressure tank, not a new pump.
- Do I need to prime a submersible well pump?
- No. A submersible sits underwater and is primed by definition. Priming applies to jet pumps, which sit above ground and pull water up. If a submersible behaves as though it has lost prime, the water level has probably fallen to the intake.
- Can a well pump be too big?
- Yes, and unlike a pressure tank this one matters. An oversized pump draws the well down faster than it recharges, risking running dry, and it short-cycles against the tank. Bigger is not safer here — matched is.
- How much does it cost to run a well pump?
- A 1 HP pump draws roughly 0.75 to 1 kW while running. A typical household running it an hour a day lands near $3 to $8 a month at average US electricity prices. A sharp rise usually means a leak or a failing tank making the pump run far more than it should.