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What size pressure tank does your well pump need?

Find the drawdown of a bladder pressure tank from the pump's cut-in and cut-out pressures, and the tank size for a minimum pump run time.

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Minimum tank volume

38.8

Choose the next size up and check the tank's own drawdown table
Drawdown factor
25.8%
Drawdown needed per cycle
10
Air precharge (tank empty of water)
38

What a pressure tank does

A private well pump does not run every time you open a tap. It fills a pressure tank, in which a rubber bladder or diaphragm separates the water from a cushion of compressed air. As you use water, the air pushes it out. When the pressure falls to the cut-in pressure, the pressure switch starts the pump; when it reaches the cut-out pressure, the pump stops.

Drawdown

The water delivered between the cut-out and cut-in pressures is the drawdown. Because air compresses according to Boyle’s law, the drawdown is only a fraction of the tank’s volume:

Drawdown factor = (precharge + atm) × (1 ÷ (cut-in + atm) − 1 ÷ (cut-out + atm))

with pressures in absolute terms (atmospheric pressure, about 14.7 psi, added to the gauge readings). The precharge, the air pressure with the tank empty of water, is usually set 2 psi below the cut-in pressure.

Pressure switch Typical drawdown factor
20/40 psi about 0.34
30/50 psi about 0.30
40/60 psi about 0.26
50/70 psi about 0.23

Sizing the tank

Most pump makers recommend that the pump run at least one minute each time it starts, sometimes two minutes for larger pumps. The drawdown needed is then the pump flow rate × the minimum run time, and:

Tank volume = drawdown needed ÷ drawdown factor

Example

A 10 gpm pump on a 40/60 switch, with a 38 psi precharge, has a drawdown factor of about 0.26. For a one-minute run time, it needs 10 gallons of drawdown, so a tank of at least 39 gallons. A 44-gallon tank would be a common choice.

Maintenance

  • Check the precharge once or twice a year with the pump off and the water drained from the tank, using a tire gauge on the air valve.
  • If water comes out of the air valve, the bladder is ruptured and the tank must be replaced.
  • Rapid on-off cycling of the pump is the classic sign of a waterlogged tank or a lost precharge.

Variable-speed pumps

Constant-pressure systems with a variable-speed drive adjust the pump speed to demand and can work with a much smaller tank. Follow the manufacturer’s sizing for those systems.

Frequently asked questions

What is pressure tank drawdown?

The amount of water the tank delivers between the moment the pump stops at the cut-out pressure and the moment it starts again at the cut-in pressure. It is much smaller than the tank's total volume, often only 25 to 35%.

Why does tank size matter?

Each start wears the pump motor and the pressure switch. A tank large enough to give the pump at least about one minute of run time per cycle prevents short cycling and extends the pump's life.

What should the air precharge be?

Usually 2 psi below the cut-in pressure, measured with the tank empty of water and the pump off. For a 40/60 switch, set the precharge to 38 psi. Check it once or twice a year with a tire gauge.

What are the signs of a failing pressure tank?

The pump clicking on and off rapidly, pulsing water pressure, or water coming out of the air valve when you press it, which means the bladder is torn. A waterlogged tank should be replaced.

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Results are estimates provided for information purposes. They do not replace professional advice.

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