What size well pump do I need?

What size well pump you need is a matching problem. The pump has to deliver enough water for your household's busiest moments, lift it from the water level in the well up to your pressure tank at the pressure you want, and do all of that without pumping water out of the well faster than the well can refill. This page sets out the numbers installers use, with a worked example, so you can follow a quote and ask good questions.

Step 1: How much flow does your home need?

Start with peak demand in gallons per minute (GPM). Goulds Water Technology's residential sizing table gives the minimum pump size needed to meet peak demand without supplemental supply: Goulds Water Technology: Technical Data and Pump Fundamentals [GOULDS-TECH source note]

The same document lists typical fixture flows, such as about 5 GPM for a shower or tub, 3 GPM for a kitchen sink and 5 GPM for a washing machine, plus outdoor uses like a garden hose or sprinkler. If you irrigate or water livestock, add those flows on top.

Extension planning guides land in the same range. Nebraska Extension recommends a minimum of 10 GPM for a 2-bedroom, 2-bath home, plus 2 GPM for each additional bedroom and each additional bath, and says the system should meet peak demand for two hours. UNL Extension: Private Drinking Water Wells, Planning for Water Use For the household-demand side of the question, see how many GPM a well should produce. [UNL-G2149 source note]

Step 2: Check the flow against the well's yield

The pump can only deliver what the well can supply. Penn State Extension defines yield as the most a well can be pumped without lowering the water below the pump intake, and says the pump's rated capacity should be slightly less than the yield. Penn State Extension: Using Low-Yielding Wells National Driller states the rule directly: never install a pump that has a greater capacity than the well; if peak demand is higher than the well can supply, size the pump to the well and make up the shortfall with storage. National Driller: Tips for Properly Sizing a Submersible Water Well Pump [PSU-LOWYIELD source note] [ND-PUMPSIZE source note]

That is why a yield test matters before you buy a pump. If the well falls short of your peak flow, a storage tank and booster pump lets a smaller well pump fill storage slowly while the booster meets the peak.

Step 3: What is the total dynamic head?

Head is the pump's workload expressed in feet of water. NDSU Extension defines total dynamic head as the sum of the static head, pressure head, friction head and velocity head, and notes that a 2.31-foot column of water equals 1 psi; in a well, the lift is measured from the pumping water level, not the top of the water when the pump is off. NDSU Extension: Irrigation Water Pumps Goulds gives the same conversion: discharge head in feet equals discharge pressure in psi times 2.31. Goulds Water Technology: Technical Data and Pump Fundamentals [NDSU-AE1057 source note] [GOULDS-TECH source note]

For a typical home with a submersible pump, add four parts:

1. Pumping level: the depth from the wellhead to the water level while the pump runs at your target flow. It comes from the well log or yield test.
2. Lift above ground: the height from the wellhead to the pressure tank, plus any rise to the house.
3. Pressure: the average pressure switch setting in psi, times 2.31. A 40 to 60 psi switch averages 50 psi, or about 116 feet.
4. Friction: losses in the drop pipe and service line, which grow with flow, pipe length and smaller pipe sizes. Installers read these from friction tables.

Step 4: Read horsepower from the pump curve

With the flow and the head in hand, the installer picks a pump whose performance curve delivers your target GPM at your total dynamic head, close to its best efficiency. Goulds gives the underlying formula: brake horsepower equals GPM times head times specific gravity, divided by 3,960 times pump efficiency. Goulds Water Technology: Technical Data and Pump Fundamentals There is no reliable chart of horsepower by house size, because the same flow needs much more power from a deep pumping level than a shallow one. [GOULDS-TECH source note]

A worked example

Take a 3-bedroom, 2-bath home on a well whose yield test showed a pumping level of 150 feet at 12 GPM, with the pressure tank 10 feet above the wellhead and a 40 to 60 psi switch.

Goulds' table puts the minimum flow for two baths at 14 GPM, and the well tested at 12 GPM. Following the rule above, the pump should be sized to about 12 GPM at roughly 290 feet of head, with storage added if the household needs more at peak. The friction figure here is only an illustration; your installer will calculate it for your pipe and flow.

Where does the pump go in the well?

Grundfos's installation guidance says the pump must be installed below the dynamic water table, the level while pumping, so it does not run dry. Grundfos: Pump sizing and installation in domestic groundwater applications Leave room for water levels that fall in a dry season, and protect a pump in a marginal well with a dry-run or low-water cutoff device. No general chart can give you the right setting depth; it comes from the well log, the yield test and your installer's judgment. [GRUNDFOS-SIZING source note]

Before you buy

1. Get the well log and a recent yield test with the pumping level at a stated rate.
2. Count your fixtures and outdoor uses, and decide your target peak flow.
3. Ask the installer for the total dynamic head calculation and the pump curve they chose from.
4. Confirm the pump's flow at that head does not exceed the well's tested yield.
5. Ask what the quote includes: pump, motor, wire, drop pipe, pressure tank and controls. See is the well pump included in a drilling quote and planning a pump for a new well.