Drawdown, not nominal gallons, is the whole game
The most expensive pressure tank mistake is reading the gallon rating on the label and assuming that is what the tank gives you. It isn't. A well pressure tank calculator that asks only for nominal gallons will mislead you, because inside a modern bladder or diaphragm tank, water is stored against a cushion of compressed air; only the water that pushes back out as the pressure falls from cut-out to cut-in is usable. That usable volume is the drawdown, and it is only about a quarter to a third of the nominal size. A 44-gallon tank on a 30/50 switch may deliver only ten to fourteen gallons before the pump kicks on again. Size by nominal gallons and you will quietly buy a tank that is far too small.
So the right sizing question is never “how many gallons is the tank” but “how much drawdown does it give at my pressure settings, and is that enough.” Asking what size well pressure tank you need is really asking what drawdown you need, and a well pressure tank size follows from that. This calculator answers both directions: the tank size for a target drawdown, or the drawdown of a tank you already have. It works as a well tank sizing tool the way a drawdown-based pressure tank sizing chart would, but from your own numbers.
Sizing to keep the pump running — the cycling problem
A pressure tank exists to stop the pump short-cycling. Every time a pump motor starts it draws several times its running current and heats up; a pump that flicks on and off every few seconds wears out far ahead of its time, taking the pressure switch contacts with it. The fix is to make sure the tank holds enough water that, at your peak draw, the pump runs for at least a full minute each time it starts. That gives the simple rule for required drawdown:
A 10 gpm pump at the one-minute minimum needs 10 gallons of drawdown; target two minutes and it needs 20. You cannot really oversize a pressure tank — a bigger tank just means fewer, longer cycles — but you can very easily undersize one, which is the failure this sizing step prevents.
Turning drawdown into tank size: Boyle's law
How much of the tank is usable depends entirely on the cut-in and cut-out pressures, through Boyle's law for the trapped air. Working in absolute pressures (gauge + 14.7 psi), with the air pre-charge set 2 psi below cut-in:
The fraction is the share of the tank's volume that comes out between cut-out and cut-in. For a 40/60 setting it works out to about 0.26 — roughly a quarter of the tank is usable — and for 30/50 it is similar. A tighter pressure band gives a smaller fraction and so needs a bigger tank for the same drawdown, which is one reason wide bands are easier on the system.
Worked example, every step shown
A 10 gpm pump on a 40/60 psi switch, sized to the one-minute minimum:
So this system wants about a 39-gallon nominal tank — round up to the next standard size. The one-to-two-gallon-per-GPM rule of thumb lands in the same neighbourhood, which is a good sanity check. Load the example with ?gpm=10&cutin=40&cutout=60&runtime=1.
Cut-in, cut-out and the pre-charge that ties them together
Cut-in is where the pressure switch starts the pump; cut-out is where it stops. The two common residential pairs are 30/50 and 40/60, each a 20 psi span. The pair you actually run is not a detail — size for 30/50 when your switch is really set to 40/60 and the drawdown comes out wrong. Whatever the setting, the air pre-charge belongs 2 psi below cut-in (28 psi for 30/50, 38 psi for 40/60), set with the tank empty and the pump off. Get the pre-charge wrong and even a correctly sized tank won't deliver its rated drawdown, and the pump short-cycles anyway.
| Switch setting | Pre-charge | Drawdown fraction |
|---|---|---|
| 30 / 50 psi | 28 psi | ≈ 0.26 |
| 40 / 60 psi | 38 psi | ≈ 0.26 |
| 20 / 40 psi | 18 psi | ≈ 0.30 |
| 50 / 70 psi | 48 psi | ≈ 0.23 |
Use the pump's real flow, not its horsepower
The flow you enter should be the pump's actual output at your pressure, not a number guessed from horsepower. A 1 HP pump is not automatically 10 gpm — its real flow depends on where its curve sits at your total dynamic head. If you don't know the figure, measure it: with the pump running, time how long it takes to fill a known container from an outdoor tap and convert to gallons per minute. To work out the head and flow your pump should be delivering in the first place, the well pump sizing calculator is the companion to this one.
Pressure tank sizing FAQ
What size pressure tank do I need?
Size by usable drawdown, not the nominal gallons on the label — and the same answer covers what size water pressure tank you need, since the physics is identical. First the drawdown: pump GPM × minimum run time, with one minute the usual floor. Then divide by the drawdown fraction your pressure settings allow. A 10 gpm pump on a 40/60 switch needs about 10 gallons of drawdown, and at a fraction near 0.26 that is roughly a 39-gallon tank. The calculator does both steps from your own numbers.
Why isn't a 40-gallon tank's drawdown 40 gallons?
Because most of the tank stays full of compressed air. Water is stored against an air cushion, and only the water pushed out as pressure falls from cut-out to cut-in is usable — the drawdown. That is roughly a quarter to a third of the nominal volume, so a 40-gallon tank typically gives only about 10–14 gallons. Sizing on nominal gallons is the most common pressure tank mistake; this tool sizes on drawdown.
How do I calculate pressure tank drawdown?
Drawdown is the tank's total volume times a fraction from Boyle's law: with pre-charge 2 psi below cut-in, fraction = P_pre_abs × (1/P_cutin_abs − 1/P_cutout_abs), where each absolute pressure is gauge + 14.7. For a 40/60 switch the pre-charge is 38 psi and the fraction is near 0.26, so about a quarter of the tank is usable. Tighter bands give a smaller fraction and need a bigger tank.
How big a pressure tank for a 10 GPM pump?
Drawdown = flow × run time. At 10 gpm and a one-minute minimum that's 10 gallons. On 40/60 the fraction is about 0.26, so the tank is about 10 ÷ 0.26 ≈ 39 gallons; round up. Target two minutes for longer pump life and the drawdown doubles to 20 gallons and roughly doubles the tank. The one-to-two-gallon-per-GPM rule of thumb agrees.
What is the difference between cut-in and cut-out pressure, and what should they be?
Cut-in starts the pump; cut-out stops it. Common pairs are 30/50 and 40/60, a 20 psi span each. The pair you actually run matters — don't assume 30/50 if your switch is set to 40/60, or the drawdown and tank come out wrong. The tool has both presets and custom, and the pre-charge should be 2 psi below your cut-in.
What should the air pre-charge be in a well pressure tank?
Set it 2 psi below the pump's cut-in pressure, checked with the tank empty and the pump off — 28 psi for a 30/50 switch, 38 psi for 40/60. A wrong pre-charge means the tank won't deliver its rated drawdown and the pump short-cycles even though the tank is the right size on paper. The tool reports the correct pre-charge for your cut-in.
Why does the pump need to run at least a minute per cycle?
Each start draws several times the running current and heats the motor, so short-cycling — starting and stopping every few seconds — wears it out fast. At least one minute of run time per cycle lets the motor settle and cool, which is why drawdown is set to flow times run time. A bigger tank gives more drawdown and fewer cycles; you can't really oversize a pressure tank, only undersize it.
Does the pump flow here come from the pump's horsepower?
No — use the pump's actual flow at your pressure, not its horsepower. A 1 HP pump isn't automatically 10 gpm; the real output depends on its curve at your head. If you don't know it, measure it by timing how long the pump takes to fill a known container. To work out the head and flow in the first place, use the well pump sizing calculator.
Is this pressure tank sizing calculator free and private?
Yes. It is free, needs no signup or download, and runs entirely in your browser, so nothing you enter leaves your device. You can copy a shareable link that reopens the calculator with your pump flow, cut-in, cut-out and run time already filled in.