Required Pump GPM & Peak Demand Calculator
Estimate the peak flow (in gallons per minute) your well pump needs to serve, based on how many fixtures could run at the same time. Enter a count and a per-fixture flow — the default 1.5 GPM is a planning convention you can change.
Calculator
Counting 8 fixtures at 1.5 gpm each, plan for a pump delivering about 12.0 gpm at peak demand. The per-fixture figure is a labeled planning convention — the well’s own yield can be the real limit.
“What size well pump do I need?” usually comes down to peak demand: the flow your plumbing needs during its busiest few minutes, not the total water you use in a day. This calculator estimates that peak from the number of fixtures that could run at once, so you can pick a pump — and, just as important, check it against what your well can actually produce.
Formula
Peak demand is the number of fixtures that can run at the same time multiplied by a planning flow per fixture:
required GPM = fixtures × GPM per fixture
The per-fixture figure (about 1.5 GPM) is a labeled planning convention, not a code value. It stands in for a full fixture-unit demand method: rather than assuming every tap in the house is open, you count the handful of fixtures realistically running together at a peak moment (a shower, the kitchen sink, a toilet refill, a washer) and size the pump for that.
Worked example
A household expects up to 8 fixtures drawing water at a busy moment, at 1.5 GPM each:
8 × 1.5 = 12 GPM
So plan for a pump and system that can deliver roughly 12 GPM at peak demand. Remember this is the flow the plumbing wants — the well itself may not sustain it. If a yield test (see the flow-rate test) shows the well only makes 5 GPM, the well’s recovery, not the pump, is the real limit, and you size storage to bridge the gap.
Peak flow vs. daily volume
Peak demand sizing balances two different numbers that people often confuse. The first is peak flow — the GPM the pump must push during the busiest few minutes of the day. The second is daily volume — the total gallons the household draws over 24 hours, which the well must be able to recover. A pump can meet a high peak flow for a short burst; whether the well can keep up over a day depends on its sustainable yield.
Because of that, treat this result as the demand target, then reconcile it against the well’s tested yield. When peak demand is higher than the well can produce, the standard fixes are a larger pressure tank or an intermediate storage/cistern tank that fills slowly during quiet hours and delivers a strong burst on demand. A driller or pump installer can confirm both the demand and the yield, and choose a pump from its performance curve at your total dynamic head.
Rules of thumb worth knowing: a small home is often sized around 6–12 GPM, a larger home with more bathrooms toward 12–20 GPM, and outdoor irrigation or livestock can dominate the peak entirely — a single sprinkler zone can want more flow than the whole house. If you irrigate, size the peak around the largest zone rather than the indoor fixtures.
Frequently asked questions
What size well pump do I need?
Count the fixtures that could realistically run at the same time during a busy moment and multiply by about 1.5 GPM each. Eight simultaneous fixtures point to roughly a 12 GPM pump. Then check that your well can actually recover that much water over a day — if it cannot, you add storage rather than a bigger pump.
Is 1.5 GPM per fixture the right number?
It is a common planning convention, not a fixed rule. Low-flow fixtures draw less; a rain-head shower or a filling tub draws more. If you know your fixtures, adjust the per-fixture flow up or down. The calculator lets you override the 1.5 GPM default with your own figure.
Why does the well's yield matter more than the pump?
A pump can only move water that is in the well. If peak demand is 12 GPM but the well only sustainably yields 5 GPM, the well’s recovery is the true ceiling. You either accept lower simultaneous use, or add a storage/pressure tank that fills during quiet periods and covers short bursts of high demand.
Does irrigation change the answer?
A lot. Outdoor irrigation and livestock watering often set the real peak, because a sprinkler zone can want more flow than every indoor fixture combined. If you irrigate, size the peak around the largest single zone and run zones one at a time.