How to size a well pump and pressure tank

By Francesco Zinghinì

Number 1: peak demand (GPM)

The pump has to deliver enough flow when several fixtures run at once. Peak demand is estimated from the number of fixtures that might run together and how much each draws:

peak demand (GPM) = fixture count × gpm per fixture

The per-fixture figure is a planning convention — about 1.5 gpm each is a common starting point. Eight fixtures at 1.5 gpm gives 8 × 1.5 = 12 gpm of peak demand. Work yours out with the required pump GPM tool, and check typical fixture flows on the fixture flow table. One crucial caveat: your well’s own yield can be the real limit — there is no point sizing a pump for 12 gpm if the well only sustains 8. Confirm yield with a flow-rate test first.

Number 2: total dynamic head (TDH)

The pump must lift water from the pumping level up to the house and into the pressurized system. That total lift, in feet, is the total dynamic head:

TDH (ft) = pumping water level + pressure head (psi × 2.31) + friction loss

For a 180 ft pumping level, a 20 psi target (20 × 2.31 = 46.2 ft) and 15 ft of friction, TDH ≈ 241 ft. TDH plus GPM is what a pump is actually selected against — a pump curve shows the flow a given pump delivers at a given head. The total dynamic head & HP tool computes TDH and gives an informational horsepower tier, but the real selection comes off the manufacturer’s curve.

Number 3: pressure-tank size from drawdown

The pressure tank’s job is to deliver water between pump starts so the pump is not switching on for every cup of water. The usable water it delivers per cycle is the drawdown, and it depends on how long you want the pump to run per cycle and the pump’s flow:

drawdown (gal) = run time (min) × pump GPM

But a tank only gives up a fraction of its total volume as drawdown, set by the pump’s cut-in / cut-out pressure through an acceptance factor (20/40 ≈ 0.20, 30/50 ≈ 0.30, 40/60 ≈ 0.40):

tank size (gal) = drawdown ÷ acceptance factor

Worked example

Aim for a 1.5-minute minimum run time with a 10 gpm pump: drawdown = 1.5 × 10 = 15 gallons. On a 30/50 switch (acceptance factor 0.30), tank size = 15 ÷ 0.30 = 50 gallons. The pressure-tank sizing tool does this for any settings, and the acceptance factors are on the pressure-tank drawdown table.

Why run time matters: short-cycling

If the tank is too small, the pump starts and stops rapidly — short-cycling — which wears out the motor and controls fast. Sizing the tank to a minimum run time (often about 1 minute for smaller pumps, more for larger) is what protects the pump. A wider pressure band (40/60) squeezes more drawdown out of each gallon of tank, which is why the switch setting changes the tank size.

Flow versus storage: two ways to meet demand

When peak demand outruns what the well can sustain, the instinct is to reach for a bigger pump — but that is often the wrong fix. If the aquifer only yields, say, 5 gpm and your peak demand is 12 gpm, a larger pump will simply draw the well down and risk running dry. The better answer is usually storage: a larger pressure tank, or a separate holding/atmospheric storage tank that the well fills slowly and steadily while a second (booster) pump delivers the household’s peaks. Storage decouples what the well produces over a day from what the house demands in bursts, letting a modest, steady yield serve a spiky demand.

This is why yield, demand and tank sizing have to be considered together rather than one at a time. A tested yield well below peak demand is not a dead end — it is a signal to design around storage. Compare your tested yield, your peak demand and your daily use side by side: if daily production comfortably exceeds daily use but peak demand exceeds instantaneous yield, storage — not horsepower — is the lever to pull.

Why oversizing a pump backfires

It is tempting to buy “a little extra” pump for headroom, but with wells bigger is often worse. A pump sized above the well’s sustainable yield will draw the water level down faster than the aquifer refills, and can pull the pumping level below the pump intake — running it dry, which is one of the fastest ways to destroy a submersible motor. An oversized pump also drives harder pressure swings and, paired with a too-small tank, can worsen short-cycling rather than help it. The right pump is matched to two ceilings at once: the well’s tested yield and the actual peak demand of the house, whichever is lower. More horsepower cannot manufacture water the aquifer does not have, so when demand outruns yield the answer is storage, not a bigger motor — exactly the flow-versus-storage tradeoff above.

Putting it together, and when to defer

Start with yield (what the well can give), set peak demand against it, compute TDH for the pump selection, then size the tank from drawdown and your switch. Compare your demand to daily household water use for a reality check. These are planning conventions — a pump installer sizes the final pump from the actual pump curve and your tested well yield, not from a rule of thumb. Use the tools to understand and budget, and let the installer make the final call.