Well Pressure Tank Sizing Calculator
Enter the pump flow, pressure switch settings, and tank precharge to estimate the minimum nominal tank volume. You can also enter an existing tank size to see its drawdown, runtime, and fastest theoretical cycling rate.
Minimum nominal tank volume33.87
- Required drawdown
- 10
- Usable drawdown factor
- 0.3%
- Precharge check
- Precharge is close to the common target of 28 psi.
- Runtime guidance check
- Selected runtime meets the 1-minute Water Systems Council guidance for this pump size.
Select a tank whose published drawdown rating meets or exceeds the required drawdown.
Multiple identical tanks can be connected together; add their nominal volumes and published drawdown capacities.
How to use this calculator
- Enter the delivered pump capacity in gallons per minute near the pressure range you use.
- Choose the minimum pump runtime required by the pump or motor manufacturer.
- Enter the cut-in, cut-out, and measured precharge pressures.
- Leave existing tank volume at zero for a new size, or enter the nominal tank volume to check an installed tank.
How the tank size is calculated
A captive-air pressure tank stores only part of its nameplate volume as usable water. The bladder or diaphragm separates air from water, and the compressed air follows the pressure in the water system. That means a 40 gallon tank does not deliver 40 gallons between pump starts; the drawdown depends on precharge, cut-in pressure, and cut-out pressure.
The calculator first converts gauge pressures to absolute pressure by adding standard atmospheric pressure, 14.7 psi. Let Q be pump capacity in GPM, t be the desired minimum runtime, Pc be precharge, Pi be cut-in, and Po be cut-out. The required drawdown is:
Dreq = Q x t
The water-filled fraction of the tank at an absolute system pressure P is:
W(P) = max(0, 1 - P0 / P)
where P0 = Pc + 14.7. The usable drawdown factor between cut-out and cut-in is:
F = W(Po + 14.7) - W(Pi + 14.7)
The minimum nominal tank volume is then:
Vreq = Dreq / F
What changes the answer most
Pump capacity and runtime set the amount of water the tank must deliver before the pump restarts. A 10 GPM pump with a one minute runtime needs 10 gallons of usable drawdown; a 20 GPM pump with a two minute runtime needs 40 gallons. The pressure settings decide how much nominal tank volume is needed to get that drawdown.
Higher cut-out pressure usually reduces drawdown factor because the air cushion is held at a higher pressure. A precharge set too high also reduces drawdown, and a precharge at or above cut-out leaves no usable water storage. The common field target for many bladder tanks is 2 psi below cut-in, measured with the pump off and water pressure relieved.
Checking an existing tank
If you enter an existing tank volume, the calculator multiplies that volume by the drawdown factor to estimate available drawdown. It then divides by pump GPM to estimate zero-demand pump runtime. For steady water use, cycling is fastest near one-half of pump capacity, so the maximum theoretical starts per hour are estimated as 15 x Q / D.
What this leaves out
Published manufacturer drawdown tables should control final tank selection because tank construction and ratings vary. This calculator assumes a captive-air bladder or diaphragm tank, a conventional pressure switch, and a pump flow rate that is representative across the operating range. It does not model galvanized air-over-water tanks, variable-speed constant-pressure systems, well yield, local plumbing rules, or pump-motor limits.
Worked example
For a 10 GPM pump, a desired one minute runtime, a 30/50 psi pressure switch, and 28 psi precharge, the required drawdown is 10 gallons. Using absolute pressures, the usable drawdown factor is about 29.53%, so the minimum nominal tank volume is 10 ÷ 0.2953, or 33.9 gallons.
If the existing tank is 44 gallons with the same settings, estimated drawdown is 13.0 gallons. With no simultaneous water demand, that gives about 1.30 minutes of pump runtime, and the fastest theoretical cycling rate is about 11.5 starts per hour.
Common questions
What size pressure tank do I need for a 10 GPM well pump?
With a one minute runtime and common 30/50 psi settings with 28 psi precharge, the minimum nominal tank volume is about 34 gallons. If the pump or motor manufacturer calls for a longer runtime, multiply 10 GPM by that runtime first, then size the tank from the resulting drawdown requirement.
Why is tank drawdown smaller than the stated tank volume?
The stated tank volume includes the air space and water space inside the tank. Only the water released as pressure falls from cut-out to cut-in is usable drawdown. Higher pressures and higher precharge settings can make that fraction much smaller than the nameplate volume.
Should pressure-tank precharge be 2 psi below cut-in?
That is the common setting for many bladder and diaphragm well tanks, but the tank manufacturer instructions should control. Measure precharge only after turning the pump off, isolating or draining the tank, and relieving water pressure. Precharge at or above cut-in reduces usable drawdown and can cause poor delivery before the pump starts.
How do 20/40, 30/50, and 40/60 settings change tank size?
At the same 2 psi below cut-in precharge, lower pressure ranges usually give a larger drawdown fraction. For a fixed pump runtime target, a 20/40 setup needs less nominal tank volume than a 30/50 setup, and a 40/60 setup needs more. The tradeoff is system pressure, not pump capacity.
Will a larger pressure tank increase water pressure?
No. The pressure switch, pump capability, plumbing losses, and elevation determine water pressure. A larger tank mainly stores more drawdown, which gives longer pump runs and fewer starts.
Can I connect two well pressure tanks together?
Yes, multiple compatible tanks can be piped together when the installation follows manufacturer and plumbing requirements. Their nominal volumes and published drawdown capacities may be added, so two identical 44 gallon tanks behave like about 88 gallons of nominal tank volume.