How it works
A pressure tank stores water so the pump doesn't start and stop too often. Manufacturers size tanks from the drawdown, the water delivered between cut-out and cut-in, needed to keep the pump running a minimum time.
The drawdown factor comes from Boyle's law using absolute pressures. With the precharge equal to cut-in at 30/50 psi it works out to about 0.31, close to the 0.30 in Pentair's sizing example. A precharge 2 psi below cut-in gives slightly less drawdown.
Variable-speed (constant-pressure) pump drives use much smaller tanks. Follow the drive manufacturer's minimum tank size instead.
The formula
- Drawdown needed = pump GPM × minimum run time (min)
- Drawdown factor = (precharge + 14.7) × [1 ÷ (cut-in + 14.7) − 1 ÷ (cut-out + 14.7)] (absolute pressures, Boyle's law)
- Minimum tank volume = drawdown ÷ drawdown factor
How to use this calculator
- Enter your pump's flow rate.
- Enter the manufacturer's minimum run time.
- Enter your pressure switch settings and tank precharge.
- Pick a tank whose total volume (not drawdown) is at least the result.
Frequently asked questions
Is tank size the same as drawdown?
No. A tank's drawdown is only a fraction of its total volume, for example roughly a third at 30/50 psi. Tanks are sold by total volume.
What if I live at high elevation?
Atmospheric pressure is lower at altitude, which changes the factor slightly. The calculator uses the sea-level value of 14.7 psi.
Sources
- Pentair WellMate pressure tank catalog: tank sizing: Drawdown = pump GPM × minimum run time; tank volume = drawdown ÷ Boyle's-law drawdown factor (30/50 psi ≈ 0.30)
- Nebraska Dept. of Water, Energy & Environment: Water Systems Sizing and Selection: 1 GPM per outlet, 3 GPM per outlet used at once; pressure × 2.31 ft; precharge 2 psi below cut-in
Sources last checked Oct 21, 2026Written and reviewed by the Tapped Out team against the sources above.How we calculate