Well Pump Size Calculator

Updated 17 October 2026

A two-bathroom home needs a pump that delivers at least 14 GPM, and with a 170 ft pumping water level and a 60 psi cut-out that flow has to be delivered against about 329 ft of head. The 14 GPM figure is the Goulds Water Technology minimum pump size for 2 to 2.5 bathrooms, and the 2.31 feet per psi conversion comes from the same manual and from Texas A&M AgriLife Extension. Horsepower is the last thing you pick, not the first, because it follows from flow and head.

Most pump-size advice stops at “match the horsepower to the well depth”. That rule hides the pressure switch, the pipe, and the climb to the house, which together can add more head than the well itself. This calculator adds every term, shows each one in a results table, and tells you when the answer points to something other than a bigger pump: a well that cannot keep up, or a jet pump asked to lift from too deep.

Well Pump Size Calculator

How the Calculation Works

Four steps, in order. The calculator does exactly this and nothing else.

1. Find the flow. By bathrooms, Goulds gives the minimum pump that meets a normal seven-minute peak: 7 GPM for one bathroom, 10 for one and a half, 14 for two to two and a half, and 17 for three to four. By fixtures, Goulds counts one gallon per minute for every fixture, hose bibs included. Irrigation and other continuous uses are added on top.
2. Find the lift. Static water level plus drawdown gives the pumping water level, the depth the pump actually pulls from. Static level alone understates it.
3. Convert pressure to feet. Each psi equals 2.31 feet of head, so a 60 psi cut-out is 138.6 ft. Goulds’ own worked example sets the switch from the pressure needed at the faucet; this tool uses the cut-out, the highest pressure the pump has to reach, so the answer is conservative.
4. Add friction, then add it all up. Friction is the Goulds loss per 100 ft for your pipe size at your flow, multiplied by your pipe run. TDH is lift plus climb plus pressure head plus friction.

Worked Example: Two Bathrooms, 170 ft to Water

A two-bathroom house sits 10 ft above its well. Static water is at 150 ft and drops 20 ft when the pump runs. The switch is 40/60 and 300 ft of 1-1/4 in plastic pipe runs from the pump to the pressure tank. These are the calculator’s default entries, so you can reproduce every line.

The pressure head is the largest single term after the water level: 138.6 ft of the 328.5 ft comes from the switch setting. That is why two houses with the same well depth can need different pumps.

What Pipe Size Does to the Answer

Friction is the term you control cheaply. Using the same example at 14 GPM, the pipe size changes the head the pump has to overcome.

Moving from 1 in to 1-1/4 in pipe removes about 29 ft of head, and the band does not change in this case. On a house closer to a band boundary it does. Goulds makes the same point about suction pipe: a smaller pump on larger pipe can do the work of a larger pump on smaller pipe, and larger pipe costs far less than a larger pump.

Bathrooms to Flow: the Goulds Numbers

Goulds states these as the minimum pump that meets peak demand “without supplemental supply”. A larger pressure tank stores some of the peak. The tool does not model the tank, so it sizes to the table. Goulds also notes that 10 to 12 GPM is good practice for an average home where the well can supply it, and that two faucets running together need about 6 GPM at 3 GPM each.

When the Well, Not the Pump, Sets the Limit

A pump cannot deliver more than the well gives back. Goulds describes a low-yield well of 1 to 2 GPM that cannot supply the 10 to 12 GPM an average home asks for: pump at 12 GPM into a well that refills at 2 GPM and the pump runs dry. If you enter a well yield below the required flow, the calculator puts a warning above the result, so the band is not mistaken for a green light.

There is a way out for some deep wells. Water stored in the casing counts: Goulds puts a 4 in casing at about 0.65 gallons per foot and a 6 in casing at about 1.4 gallons per foot, so a deep well with a high static level can hold hundreds of gallons. A low-water cut-off or a pump sized below the yield protects the pump. The yield comes from the driller’s report. Once you know what to buy, our well pump replacement cost guide shows what a replacement runs.

Jet Pump or Submersible

The calculator checks one hard limit for a jet pump. Atmospheric pressure of 14.7 psi lifts water 33.9 ft at sea level, and Goulds does not recommend suction lifts past 25 ft, deducting about 1 ft of lift for each 1,000 ft of elevation above sea level. Past that depth a shallow-well jet pump stops delivering its rated flow. In that case the tool still prints a submersible horsepower band and flags the jet choice. Our submersible vs jet well pump guide covers the trade-offs, and the pressure switch guide explains what the cut-in and cut-out numbers do.

For a jet pump that passes the lift check, the tool gives flow and head but not a horsepower band. Jet pump output depends on the nozzle and venturi in each model and Goulds publishes it per model, so the honest answer is to read that model’s performance table at your flow and pressure.

What the Horsepower Band Does and Does Not Tell You

The band comes from the Goulds 5GS to 25GS residential submersible charts, which list flow by depth to water at 20, 30, 40, 50 and 60 psi. The tool reads them as printed. Depth is the pumping level plus the climb plus the pipe friction, and the pressure row is your cut-out rounded up to the next printed row. Goulds’ printed flows already allow for the friction in the drop pipe, which is why the total dynamic head above is the teaching figure and the chart reading is the selection step. The tool finds the smallest motor in any Goulds family whose recommended flow range holds your flow and that still delivers it at that depth and pressure, then shows that horsepower with the next size up. Above 60 psi the charts stop and the tool says so rather than extrapolating. It is a selection aid, not a quote.

Three things are outside it. It uses straight-pipe friction, so elbows and valves add to your real head. It reads one manufacturer’s curves, and another brand’s pump with the same horsepower can deliver more or less. And it does not size wire, the control box or the breaker, which depend on the motor, the voltage and the distance to the panel. Confirm the pump curve and the electrical spec with the maker or your installer. If you are replacing an existing pump, the how to replace a well pump walkthrough covers the pull, the pipe and the wiring.

Frequently Asked Questions (FAQs)

What size well pump do I need for my house?

Start with flow, not horsepower. Goulds gives a minimum pump of 7 GPM for one bathroom, 10 for one and a half, 14 for two to two and a half, and 17 for three to four. Then work out the total dynamic head: pumping water level, plus climb, plus 2.31 ft per psi of switch pressure, plus friction. Horsepower follows from those two numbers and the manufacturer’s pump curve.

How do you calculate total dynamic head for a well pump?

Add four terms in feet: the pumping water level (static level plus drawdown), the elevation from the well head to the house, the cut-out pressure in psi multiplied by 2.31, and the friction loss in the pipe from the pump to the tank. A pump at a 170 ft pumping level, a 10 ft climb, a 60 psi cut-out and 9.9 ft of friction faces 328.5 ft of head.

How deep can a jet pump pull water from?

Goulds does not recommend suction lifts above 25 ft at sea level for a shallow-well jet pump, and the figure drops by about 1 ft for every 1,000 ft of elevation above sea level. The 25 ft has to cover friction in the suction pipe as well as the vertical lift. Deeper water calls for a submersible or a deep-well jet system.

How many GPM does a three-bedroom house need?

Goulds sizes by bathrooms rather than bedrooms. A three-bedroom house with two bathrooms falls in the 2 to 2.5 bathroom row, which is a minimum pump of 14 GPM, and a house with one and a half bathrooms needs 10 GPM. Add the flow of any hose or sprinkler you run at the same time, about 3 GPM for a 1/2 in hose.

Does a higher horsepower pump give more water?

Not by itself. Flow falls as head rises along the pump’s curve, so a bigger motor mostly buys the ability to deliver the same flow against more head. A pump that outruns the well still runs it dry. Check the well yield first, then pick the smallest motor whose curve delivers your flow at your head.