What size well pressure tank do I need?
Sized from the drawdown your pressure switch actually delivers, rather than a rule of thumb.
The short answer
Size the tank so your pump runs at least one minute per cycle — two minutes for motors of 1 HP and above. A tank only delivers a fraction of its volume between cut-in and cut-out, and that fraction depends on the pressure switch setting: about 30% at 20/40 psi, 25% at 30/50, and 21% at 40/60.
Recommended tank
62gallon
- Drawdown you needWater actually delivered between the pump cutting in and cutting out.
- 10.0 gal
- Drawdown factor at this settingA wider pressure band stores more usable water per gallon of tank.
- 21%
- Minimum tank volume
- 47.6 gal
Can a pressure tank be too big?
Not for the pump — a larger tank means longer runs and fewer starts, which is strictly better for pump life. Going up to 86 gallons is a reasonable upgrade if you have the space and the budget.
The real limits are practical: the tank costs more, takes more floor, and on a rarely-used supply the extra stored water sits longer between changes. What you should not do is undersize — that is the failure that burns out pumps.
Drawdown is what you're actually buying
A pressure tank is not a water container. It is an air spring: a sealed air cushion above a bladder, compressed as water is pumped in and pushing water back out as taps are opened. The pump only runs when pressure falls to the cut-in setting.
Drawdown is the water actually delivered between cut-in and cut-out, and it is far less than the tank’s volume — typically a fifth to a third. That is the number that matters, and it is why a “44 gallon tank” gives you around 11 gallons of usable water, not 44.
A wider pressure band stores more usable water per gallon of tank, which is why the same pump needs a bigger tank at 40/60 than at 20/40. Most people find 40/60 more comfortable, and pay for it in tank size.
Why a minimum run time matters more than comfort
Short-cycling — the pump switching on and off rapidly — is what destroys well pumps. The damage is not from running; it is from starting. Motor start current is several times running current, and the heat it generates is shed while the motor runs. A pump that starts constantly and runs briefly never gets to cool.
A minute of run time per cycle is the accepted minimum for motors under 1 HP, and two minutes for larger ones. Sizing the tank to deliver that is the single cheapest thing you can do for pump life.
- Required drawdown (gallons) = pump flow in GPM x minimum run time in minutes.
- Tank volume = required drawdown divided by the drawdown factor for your pressure setting.
- Then round up to the next standard tank size.
The air pre-charge, and why it's usually wrong
A pressure tank needs an air pre-charge set to 2 psi below the pump’s cut-in pressure. A 40/60 switch wants 38 psi in the tank; a 30/50 switch wants 28.
This is measured with a tyre gauge on the schrader valve at the top of the tank, with the system depressurised — open a tap and drain the pressure first, or you will read water pressure rather than air charge. Tanks lose a little air over years, and a low pre-charge cuts drawdown dramatically while looking like a failing pump.
If there is no air pressure at all, or water comes out of the schrader valve, the bladder has failed and the tank needs replacing.
Frequently asked questions
- Can a well pressure tank be too big?
- Not for the pump. A larger tank means longer run times and fewer starts, which is strictly better for pump life. The real limits are cost, floor space, and that on a rarely used supply the stored water sits longer between changes. Undersizing is the failure that burns out pumps.
- What should the air pressure be in my well pressure tank?
- Two psi below the pump's cut-in pressure, measured with the system depressurised. A 40/60 switch wants 38 psi; a 30/50 switch wants 28 psi.
- Do well pressure tanks go bad?
- Yes. The internal bladder is the wear part and typically lasts 8 to 15 years. When it fails, air and water mix — you get rapid pump cycling, cloudy water, and sometimes black rubber fragments in the water. The tank is replaced rather than repaired.
- Can a well pressure tank freeze?
- Yes, and a frozen tank can split. Tanks in unheated spaces need insulation or heat, and a tank installed outdoors needs proper protection. A split tank is not repairable.
- Can a well pressure tank lie on its side?
- Most bladder tanks are designed to stand upright and should be installed that way — horizontal mounting stresses the bladder unevenly and shortens its life. Some models are specifically built for horizontal mounting; check the manufacturer's guidance rather than assuming.