Free Calculator · Published September 2026
Well Pump Size Calculator: What Size Well Pump Do I Need?
Work out the flow the house needs, the total dynamic head the pump has to push against, an approximate horsepower band and the pressure tank to pair with it. Includes the backwash flow that water treatment and water purification equipment needs, which is where most undersized pumps show up first.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
Book a DemoSize the Pump and Pressure Tank
Enter the house, the well and the run to the pressure tank. The result is a duty point, gpm at feet of head, which is how pumps are actually selected.
1. Water demand
The calculator uses the middle of each range.
Only if it runs while the house is using water.
From the driller’s log or a flow test. Leave blank if unknown.
Use the largest backwashing tank in the train. Confirm the rate on the valve spec sheet.
2. The well and the run to the house
Depth to water with the pump off.
How far the level drops under load.
Usually 10–20 ft below the pumping level.
Negative if the house sits below the well.
40/60 is common where treatment equipment drops pressure after the tank.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on common well-industry rules of thumb, Hazen-Williams friction loss for plastic pipe and Boyle’s law tank drawdown, and should be used for general planning only. Actual pump performance varies by model, so always confirm the selection on the manufacturer’s pump curve and have a licensed well or pump installer verify wiring, controls and local code requirements.
How Well Pump Sizing Works
Start with flow, and check it against the well
A well pump is chosen on a duty point: so many gallons per minute at so many feet of head. Peak demand comes first. The two common rules of thumb give similar answers for most homes:
| Bathrooms | Peak demand | Typical fixture count |
|---|---|---|
| 1 | 6–8 gpm | 6–8 fixtures |
| 2 | 8–10 gpm | 8–10 fixtures |
| 3 | 10–12 gpm | 10–12 fixtures |
| 4+ | 12–16 gpm | 12+ fixtures |
Then compare demand with the well’s recovery rate from the driller’s log. A pump should not move more water than the well gives back. If the well recovers 5 gpm and the house wants 10, the fix is storage and drawdown, not a bigger pump.
Water treatment backwash is part of the demand
Backwashing filters for iron, sulfur and sediment need a minimum flow to lift and clean the media bed, typically about 5 gpm for a 10" tank, 7 gpm for 12", 8 gpm for 13" and 10 gpm for 14", depending on the media. Softeners need less, usually 2 to 5 gpm. The backwash runs for 10 to 15 minutes, so the pump and the well both have to sustain it at operating pressure.
When they cannot, the bed never fluffs, iron stays packed in it and the filter fouls. It looks like a media problem and gets blamed on the equipment, but the cause is a pump or a well that was never sized for it. Before specifying an iron filter for well water or a water softener, check the backwash rate against the pump. The well water treatment system selector sets out which stages the water actually needs.
Total dynamic head, term by term
TDH = pumping level + elevation rise + (cut-out psi × 2.31) + friction loss
Pumping level is static level plus drawdown. It is not the pump setting depth. The pump lifts water from where the water surface sits while it runs, and setting the pump deeper only adds pipe. Pressure head is usually the largest single term on a shallow or medium well: a 60 psi cut-out is 139 ft. Friction depends heavily on pipe size:
| Flow | 1" poly | 1-1/4" poly | 1-1/2" poly |
|---|---|---|---|
| 5 gpm | 1.5 ft | 0.4 ft | 0.2 ft |
| 7 gpm | 2.9 ft | 0.8 ft | 0.4 ft |
| 10 gpm | 5.5 ft | 1.5 ft | 0.7 ft |
| 12 gpm | 7.8 ft | 2 ft | 1 ft |
| 15 gpm | 11.7 ft | 3.1 ft | 1.5 ft |
Feet of head lost per 100 ft of pipe, Hazen-Williams C = 150. At 10 gpm, 1" pipe loses nearly four times what 1-1/4" does.
Well pump sizing chart
Approximate submersible horsepower for a given flow and total dynamic head, using the same formula as the calculator: HP ≈ gpm × TDH ÷ (3,960 × 0.7). Treat it as a way to narrow the search, then read the real curve.
| Flow | 100 ft | 150 ft | 200 ft | 300 ft | 400 ft |
|---|---|---|---|---|---|
| 5 gpm | 1/2 HP | 1/2 HP | 1/2 HP | 3/4 HP | 3/4 HP |
| 7 gpm | 1/2 HP | 1/2 HP | 3/4 HP | 1 HP | 1-1/2 HP |
| 10 gpm | 1/2 HP | 3/4 HP | 3/4 HP | 1-1/2 HP | 1-1/2 HP |
| 12 gpm | 1/2 HP | 3/4 HP | 1 HP | 1-1/2 HP | 2 HP |
| 18 gpm | 3/4 HP | 1 HP | 1-1/2 HP | 2 HP | 3 HP |
Well pressure tank sizing
A tank is sized on drawdown, the water it delivers between cut-out and cut-in. Motors need a minimum run time to shed start-up heat, so the working rule is drawdown = pump gpm × minimum run time, with about 1 minute per horsepower and never less than 1 minute. Divide by the drawdown factor to get total tank volume:
| Switch setting | Pre-charge | Drawdown factor | 44 gal tank delivers |
|---|---|---|---|
| 20/40 psi | 18 psi | 0.34 | 15 gal |
| 30/50 psi | 28 psi | 0.3 | 13.2 gal |
| 40/60 psi | 38 psi | 0.26 | 11.4 gal |
| 50/70 psi | 48 psi | 0.23 | 10.1 gal |
Raising the switch from 30/50 to 40/60 costs about 10% of the tank’s drawdown. That is worth knowing on water treatment jobs, where the softener, filter and UV train can drop 5 to 15 psi after the tank and a 40/60 switch is the usual fix.
A worked example
Three-bathroom house. The well log shows 80 ft static, 40 ft drawdown and 15 gpm recovery. Pump set at 140 ft, 20 ft rise to the house, 150 ft horizontal run in 1-1/4" poly, 40/60 switch, and a 12" backwashing iron filter.
Design flow = larger of 11 gpm (3 baths) and 7 gpm (backwash) = 11 gpm
Pumping level = 80 + 40 = 120 ft
Pressure head = 60 × 2.31 = 138.6 ft
Friction = 1.74 ft/100 × 290 ft × 1.1 = 5.6 ft
TDH = 120 + 20 + 138.6 + 5.6 = 284 ft
HP = 11 × 284 ÷ (3,960 × 0.7) = 1.13 → 1-1/2 HP band, 12 gpm series
Tank = 11 gpm × 1.5 min = 16.5 gal drawdown ÷ 0.26 = 63 gal → 86 gal tank
Notice where the head comes from: more than 250 of the 284 ft is pumping level and switch pressure. Friction is small only because the pipe is 1-1/4". The same run in 1" pipe would lose about 21 ft, enough to push a borderline pump off its curve.
For water treatment dealers: size the pump before you sell the filter
Well pumps and water purification equipment get sold separately, often by different trades, and the backwash rate is where they collide. A two-minute check of the pump against the filter’s backwash rate prevents the callback where a new iron filter fouls in six months. Where the pump work crosses into plumbing, it is also a natural add-on line on the same quote. Pair this with the UV sizing calculator, which uses the same peak flow.
Recording the well yield, static level, pump and tank on every job means the next service visit starts with the numbers instead of a guess. That job history, the quote and the follow-up schedule are what software for water treatment dealers is built to hold.
Frequently Asked Questions
What size well pump do I need?
Work it out from flow and head, not horsepower. First estimate peak demand: roughly 6 to 8 gpm for a one-bathroom home, 8 to 10 for two, 10 to 12 for three and 12 to 16 for four or more, or about 1 gpm per water-using fixture. Check that figure against what the well can actually recover. Then add up total dynamic head: the pumping water level, the rise from the wellhead to the pressure tank, the cut-out pressure multiplied by 2.31, and friction loss in the pipe. Choose a pump whose curve delivers the flow at that head. A typical three-bathroom house with a 120 ft pumping level lands around 11 gpm at roughly 280 ft of head, which is usually a 1 to 1-1/2 HP pump.
How do you calculate total dynamic head for a well pump?
Total dynamic head (TDH) is the sum of four things, all in feet. Pumping level is the static water level plus the drawdown while the pump runs, not the depth the pump is set at. Elevation rise is the vertical distance from the wellhead up to the pressure tank, and it can be negative if the house sits below the well. Pressure head is the pressure switch cut-out setting multiplied by 2.31, so 60 psi is about 139 ft. Friction loss comes from the drop pipe and the horizontal run, and depends on pipe size and flow: 1-1/4 inch poly loses about 1.5 ft per 100 ft at 10 gpm, while 1 inch loses about 5.5 ft.
How do I size a well pressure tank?
Size the tank on drawdown, the water it delivers between cut-out and cut-in, not on its total volume. The target drawdown is the pump flow multiplied by its minimum run time, commonly 1 minute per horsepower with a 1 minute floor. Divide that by the drawdown factor for your pressure switch setting, about 0.29 for 30/50, 0.26 for 40/60 and 0.23 for 50/70 with the tank pre-charged 2 psi below cut-in. A 10 gpm, 1 HP pump on a 40/60 switch needs about 10 gallons of drawdown, which means a tank of roughly 39 gallons total volume, so a 44 gallon tank.
Can a well pump be too big?
Yes. A pump that moves more water than the well recovers will draw the water level down to the intake, run dry and burn out, or it will short-cycle as the pressure tank fills in seconds. Oversizing also raises the running amps and the wire size needed. If peak demand is higher than the well yield, the usual answers are a pump sized at or below the yield with a larger pressure tank, or a low-yield setup with an atmospheric storage tank and a booster pump. Dry-run or low-water protection is worth fitting whenever the pump flow is close to the yield.
Why does water treatment equipment affect well pump size?
Backwashing filters need a minimum flow to lift and clean their media bed, typically around 5 gpm for a 10 inch tank up to about 10 gpm for a 14 inch tank, and softeners need a few gpm during backwash. If the pump and well cannot supply that flow at the operating pressure for the full backwash cycle, the bed never fluffs, iron and sediment stay packed in it, and the filter fouls or channels. An undersized pump is one of the most common reasons an iron or sediment filter fails within a year. Check the backwash rate on the valve and media spec sheet and confirm the well can sustain it.
Should I use a 30/50 or 40/60 pressure switch?
30/50 is the most common factory setting and gives the most drawdown per gallon of tank. 40/60 gives better pressure at the fixtures and is often chosen when water treatment is installed, because a softener, filter and UV train can drop 5 to 15 psi between the pressure tank and the house. The trade-off is that a higher setting adds head the pump must produce, about 23 ft for every 10 psi, and lowers the drawdown factor, so the tank has to be larger. Whatever the setting, the tank pre-charge should be set about 2 psi below cut-in with the tank empty.
Quote the Pump, the Tank and the Treatment Together
Build the full well and water treatment proposal on site, keep every well’s numbers on the customer record, and put filter changes and service visits on a schedule that runs itself.
Get a Free DemoPump and tank sizes are approximate estimates from common well-industry rules of thumb and should be confirmed on the manufacturer’s pump curve by a licensed installer. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.