Pump size depends on your household's peak water demand and how deep your well is
A well pump must be large enough to deliver water at the rate your household uses it during peak times — typically morning showers and laundry running together — and powerful enough to push water up from your well depth. If the pump is too small, water pressure drops when multiple fixtures run at once. If it is oversized, it cycles on and off too often, wears out faster, and costs more to run. The right size balances flow rate (measured in gallons per minute, or GPM) against the vertical distance from the water table to your house (called total dynamic head, or TDH).
Most residential wells use either a submersible pump (sits inside the well) or a jet pump (sits above ground). Submersible pumps are rated by horsepower and GPM together; jet pumps are rated by horsepower alone but come with a flow curve that shows GPM at different depths. Both types must overcome the depth of your well plus the pressure needed to push water through your house plumbing.
Key Takeaways
- Calculate your peak water demand by adding the GPM of every fixture that might run at the same time — typically 3 to 5 GPM per bathroom, 2 to 3 GPM per kitchen sink, and 1 to 2 GPM per outdoor spigot.
- Measure the distance from your well's static water level (the resting level when no water is being drawn) to the highest fixture in your house to find your total dynamic head.
- A 1-horsepower submersible pump typically delivers 5 to 8 GPM at 100 feet of depth; a 1.5-horsepower pump delivers 8 to 12 GPM at the same depth.
- Oversizing a pump by more than 50 percent above your actual peak demand causes rapid on-off cycling that shortens the pump's life and wastes electricity.
- If your well is deeper than 25 feet, a submersible pump is usually more efficient and reliable than a jet pump.
Calculate your household's peak water demand in gallons per minute
Peak demand is the total flow when the most fixtures run at once. This is not the average — it is the worst-case moment. List every fixture that could run simultaneously: showers, toilets, washing machine, dishwasher, outdoor hose, kitchen and bathroom sinks. Assign each a flow rate based on its type.
A standard showerhead uses 2.5 GPM (federal limit since 1992). A toilet fill uses 1 to 3 GPM depending on age. A washing machine uses 3 to 5 GPM during fill. A dishwasher uses 2 to 3 GPM. A kitchen sink uses 2 to 3 GPM. A bathroom sink uses 0.5 to 1 GPM. An outdoor spigot or garden hose uses 5 to 10 GPM. Add the fixtures most likely to run together — for example, one shower, one toilet, and a washing machine, which totals roughly 6 to 8 GPM. Most homes need a pump rated between 10 and 15 GPM to handle peak demand with a safety margin.
Measure your well depth and calculate total dynamic head
You need two measurements: the static water level (the depth of water when the pump is off and no water has been drawn for several hours) and the elevation of your highest fixture above ground. Call a well contractor or check your well log if you have one on file with your county. If not, you can measure static level yourself by lowering a weighted string or tape measure down the well casing until it touches water, then marking the depth.
Total dynamic head is the static level plus the height your water must rise to reach the highest fixture in your house. If your static water level is 80 feet below the surface and your house is 20 feet above ground level, your TDH is roughly 100 feet. Add another 10 to 20 feet to account for pressure loss in the pipes and fittings — this is called friction loss. So a well 80 feet deep with a house 20 feet above grade needs a pump rated for 110 to 120 feet of head.
Match pump horsepower and flow rate to your depth and demand
Pump manufacturers publish performance curves — charts showing how many GPM a pump delivers at different head pressures. A 1-horsepower submersible pump typically delivers 8 GPM at 100 feet of head and 5 GPM at 150 feet. A 1.5-horsepower pump delivers 12 GPM at 100 feet and 8 GPM at 150 feet. A 2-horsepower pump delivers 15 GPM at 100 feet and 10 GPM at 150 feet.
Find the performance curve for the pump model you are considering. Locate your calculated TDH on the horizontal axis (the bottom). Move straight up to where it meets the pump's curve line. Move left to read the GPM on the vertical axis. That GPM is what the pump will actually deliver at your depth. It must be at least equal to your peak demand, and ideally 20 to 30 percent higher to avoid running the pump at its maximum capacity all the time.
If you are comparing a jet pump, the process is the same: find the curve, locate your head, read the GPM. Jet pumps are generally limited to wells shallower than 25 feet because they lose efficiency at greater depths. Submersible pumps work well at any depth and are the standard choice for residential wells.
Account for pressure tank size and system losses
A pressure tank stores water under pressure so the pump does not run every time you turn on a faucet. The tank's size affects how often the pump cycles. A larger tank means the pump runs less frequently, which extends its life. A smaller tank means more frequent cycling and faster wear.
Most residential systems use a 20 to 40-gallon tank. If your pump is 1 to 1.5 horsepower, a 20-gallon tank is typical. If your pump is 2 horsepower or larger, a 30 to 40-gallon tank is better. The pump should not cycle more than 4 to 6 times per hour during normal use. If it cycles more often, the tank is too small or the pump is oversized.
Friction loss in pipes also affects sizing. Longer pipe runs, smaller pipe diameter, and more fittings all increase friction loss. If your house is more than 100 feet from the well, add 5 to 10 feet to your TDH calculation. If it is more than 200 feet away, add 15 to 20 feet. This ensures the pump has enough pressure to overcome the resistance of the water traveling through the pipes.
Common sizing mistakes and how to avoid them
The most common error is oversizing the pump. A pump rated for 20 GPM when your peak demand is 10 GPM will cycle on and off rapidly, wear out the motor faster, and waste electricity. It will also cause pressure spikes that can damage fixtures and create water hammer noise in the pipes. Size the pump to your actual demand, not to a "what if" scenario.
Another mistake is ignoring the performance curve and assuming a pump's nameplate horsepower tells you the flow rate. A 2-horsepower pump at 50 feet of head delivers much more water than the same pump at 200 feet of head. Always check the curve for your specific depth. If the manufacturer does not provide a curve, ask for one before buying.
A third error is underestimating friction loss in long pipe runs. If your well is far from the house or your plumbing has many bends and fittings, add extra head to your calculation. Undersizing because you forgot friction loss means the pump will not deliver enough pressure during peak use, and you will have low water pressure when multiple fixtures run.
When to call a well contractor for professional sizing
If your well is deeper than 150 feet, if you are replacing a pump and do not know the original specifications, or if your current system has chronic low pressure or rapid cycling, a contractor can test your system and recommend the right pump. They have tools to measure static water level accurately, can run a flow test to confirm your actual peak demand, and know the performance curves of pumps currently available.
A contractor will also check whether your pressure tank is the right size and whether your plumbing has any leaks that inflate your apparent demand. This costs $150 to $300 but prevents buying the wrong pump, which costs $500 to $1500 to replace. If you are uncertain about any of your measurements or calculations, a professional assessment is worth the investment.
Frequently Asked Questions
What is the difference between static head and dynamic head?
Static head is the depth of water in your well when the pump is off and no water has been drawn. Dynamic head is the depth the pump must overcome while water is flowing — it is always deeper than static head because drawing water lowers the water level temporarily. For sizing purposes, use dynamic head, which is static level plus the height to your highest fixture plus friction loss in the pipes.
Can I use a larger pump if I install a bigger pressure tank?
A larger tank reduces cycling frequency but does not change the pump's flow rate or head rating. If your peak demand is 10 GPM, a pump rated for 20 GPM will still cycle too often and wear out faster, even with a 40-gallon tank. Size the pump to your actual demand, then choose a tank size that keeps cycling to 4 to 6 times per hour.
How do I know if my pump is oversized?
An oversized pump cycles on and off frequently — more than 6 times per hour — even during light water use. You may hear the pump start and stop every few minutes. You may also notice pressure spikes that cause banging in the pipes or water hammer sounds. If this happens, the pump is likely too large for your demand and should be replaced with a smaller one.
What happens if I undersize the pump?
An undersized pump cannot deliver enough water when multiple fixtures run at once. Water pressure drops noticeably — showers weaken, toilets fill slowly, and the washing machine may not fill properly. The pump runs continuously at maximum capacity, which shortens its life. If you experience low pressure during peak use, your pump is too small and needs to be replaced.
Do I need to know my well's gallons per minute capacity?
Your well's capacity (how much water it can produce per minute) is different from your pump's capacity. A well might produce 5 GPM but you only need 10 GPM peak demand, so you size the pump to 10 GPM. However, if your well produces only 3 GPM, you cannot install a 10 GPM pump — it will pump the well dry and run dry, which damages the motor. Ask your well contractor to test your well's production rate before sizing the pump.