Well Pump Size Calculator

Calculate the gallons per minute and total dynamic head a private well pump has to deliver, the horsepower band that implies, and the pressure tank drawdown to match.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a well pump, add the pumping water level, the lift to the highest fixture, the friction loss and the pressure head to get total dynamic head. A 12 fixture house with water 100 ft down while pumping needs 12 gpm at 283 ft of head, which is a 1 hp submersible.

Your measurements

A column or tube: volume is the circular area (from the diameter) times the height.

Fixtures in the houseCount each toilet, lavatory, tub or shower, kitchen sink, dishwasher, washing machine and hose bib. A 3 bedroom, 2 bath house is usually 12 to 14.Static water level below gradeftDepth from the top of the casing to the resting water surface, from the well log. This is not the total well depth, which is usually much deeper.Drawdown while pumpingftHow far the water level falls once the pump runs, from the driller's yield test. Where it is unknown, 20 ft is a common planning figure on a good well.Rise from wellhead to the highest fixtureftVertical rise only. A second floor bathroom in a house on a 10 ft rise above the wellhead is about 25 ft.Pressure switchThe pump has to reach the cut-out pressure, not the cut-in. A 40/60 switch means the pump works against 60 psi at the tank.Pipe run from well to tankftHorizontal drop pipe and buried line together. Friction is figured on 1 in pipe at the peak demand, which is what most residential wells use.

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Results

Peak demand: 12 gpm; 12 fixtures at about 1 gpm each, with a 7 gpm floor for a house
Total dynamic head: 283 ft; 100 ft pumping level + 25 ft lift + 20 ft friction + 139 ft for 60 psi at the tank
Horsepower band: 1 hp; Typical 4 in submersible at 12 gpm. Match the exact model to the pump curve, since curves differ between makers
Pressure tank: 40 gal nominal; 12 gpm of drawdown, one gallon per gpm, so the pump runs at least a minute per cycle. A 40/60 switch draws about 30 percent of the tank
Planning estimate: Contractor sizes the pump; Always consult a professional here: a licensed well contractor reads the well log and the yield test, and the pump curve governs the final choice

Well Pump Size formula

Peak demand (gpm) = Fixtures x 1, with a 7 gpm minimum
Pumping level (ft) = Static water level + Drawdown
Pressure head (ft) = Cut-out pressure (psi) x 2.31
Total dynamic head = Pumping level + Lift to highest fixture + Friction + Pressure head

Worked example

A ranch house with 9 fixtures on a shallow well: static level 40 ft, drawdown 10 ft, and the highest fixture 12 ft above the wellhead. Peak demand is 9 gpm. The pumping level is 50 ft, the lift 12 ft, and a 40/60 switch adds 60 x 2.31 = 139 ft of pressure head.

Friction on the 50 ft of drop pipe plus 150 ft of buried line at 9 gpm is about 12 ft. Total dynamic head is 50 + 12 + 12 + 139 = 212 ft. That is just past the 3/4 hp band, so a 1 hp submersible is the safe pick, with a 30 gallon tank giving about 9 gallons of drawdown.

Quick reference

Typical 4 in submersible horsepower by total dynamic head at about 10 gpm
Total dynamic head	Typical horsepower	Common use
Up to 100 ft	1/2 hp	Shallow well, single story, 30/50 switch
100 to 200 ft	3/4 hp	Water 60 to 100 ft down, two story
200 to 300 ft	1 hp	Water 100 to 150 ft down, 40/60 switch
300 to 400 ft	1-1/2 hp	Deep well or a long uphill run to the house
400 to 500 ft	2 hp	Deep well with high lift; verify against the curve

The two numbers a pump is chosen by

Every submersible is picked from a curve with two axes: gallons per minute and total dynamic head. Demand comes first. Counting fixtures and allowing about a gallon a minute each, with a floor of 7 to 10 gpm for any house, lands most homes between 8 and 15 gpm. That number has to fit inside the well yield from the driller's test, because no pump can deliver more than the well produces.

Head is the resistance the pump works against, and it is not the depth of the well. It is the water level while pumping, plus the rise to the highest fixture, plus friction in the pipe, plus the pressure you want at the tank converted at 2.31 feet per psi. That last term surprises people: a 40/60 switch alone is 139 feet of head.

Tank size and cycling

The pressure tank is what keeps the pump from starting every time someone washes their hands. A bladder tank holds roughly 30 percent of its nominal volume as usable drawdown between the cut-in and cut-out settings, so a 30 gallon tank gives about 9 gallons. The rule contractors use is at least one gallon of drawdown for each gallon per minute the pump delivers, which buys a minimum run of one minute per start.

Set the air charge 2 psi below the cut-in pressure with the tank empty, and check it once a year. A waterlogged tank gives no drawdown at all and the pump cycles on every faucet. Where a well yields less than the house demands, the answer is a storage tank and a booster pump, not a bigger submersible.

What this calculator leaves out

The well yield test, which caps everything, and the specific pump curve, which differs between makers. It does not size the wire and the drop cable for voltage drop, pick the control box, or check the motor starting current against the service. It also leaves out the pitless adapter, the check valves, the sanitary well cap, the water test, and the state setbacks and permits that decide where the well can be in the first place.

Mistakes that cost money

Sizing from the total well depth instead of the static water level: a 400 ft well with water at 80 ft only lifts from 80 ft plus drawdown.
Forgetting the pressure switch. A 40/60 setting adds 139 ft of head, often more than the well depth itself on a shallow well.
A tank too small for the pump: under a gallon of drawdown per gpm the pump starts every few seconds and the motor burns out.
Oversizing to be safe. A pump that outruns the well yield draws the level down to the intake, sucks air and overheats.
Setbacks are state law, not preference: commonly 50 ft from a septic tank and 100 ft from a drain field, with 12 in of casing above grade.
No check valve or a leaking one lets the drop pipe drain back, so the pump restarts against an empty column every cycle.

Key facts

One psi of pressure equals 2.31 feet of head, so a 40/60 pressure switch adds 139 feet of head at the cut-out setting.
A common planning rule is 1 gallon per minute per fixture, with 7 to 10 gpm as the floor for a house of any size.
Total dynamic head is the pumping water level plus the lift to the highest fixture plus friction plus pressure head, not the depth of the well.
A pressure tank holds about 30 percent of its nominal volume as usable drawdown, so a 30 gallon tank delivers roughly 9 gallons per cycle.
Short cycling is what kills submersible motors: a minimum run of one minute per start is the usual target, which sets the tank size.

Frequently asked questions

What size well pump do I need for a 3 bedroom house?

Most 3 bedroom houses have 12 to 14 fixtures, which is 12 to 14 gallons per minute of peak demand. With water 80 to 100 ft down and a 40/60 switch, total dynamic head lands near 250 to 275 ft, which is a 1 hp submersible on typical curves.

Is total dynamic head the same as well depth?

No. Head is measured from the water level while pumping, not the bottom of the well, and it adds the rise to the highest fixture, the friction in the pipe and the pressure at the tank. A 400 ft well with water at 80 ft lifts from 80 ft plus drawdown.

What size pressure tank goes with the pump?

Enough drawdown for a one minute minimum run: about one gallon of drawdown per gallon per minute of pump output. Since a tank gives roughly 30 percent of its nominal size as drawdown, a 10 gpm pump wants a 30 to 40 gallon tank.

Can a well pump be too big?

Yes. A pump that outruns the well yield pulls the water level down to the intake, then draws air and overheats. Match the pump to the tested yield, and use storage plus a booster where the well is slow.

Sources and references

Submersible Motor Application, Installation and Maintenance Manual, pump sizing and total dynamic head (Franklin Electric)
Well construction, casing and setback requirements (Minnesota Department of Health)
Private Water Systems, household demand and system planning (Penn State Extension)
Protect Your Home's Water, private well basics (EPA)

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