Pressure tank size
A well pressure tank stores enough water that the pump runs at least one minute every time it starts. Pick your pump's flow for the tank size at each common switch setting, or use the tank calculator for any flow.
| Pump flow | 20/40 psi | 30/50 psi | 40/60 psi |
|---|---|---|---|
| 5 gpm | 14.5 gal | 16.9 gal | 19.4 gal |
| 7 gpm | 20.3 gal | 23.7 gal | 27.1 gal |
| 10 gpm | 29 gal | 33.9 gal | 38.8 gal |
| 14 gpm | 40.6 gal | 47.4 gal | 54.3 gal |
| 17 gpm | 49.3 gal | 57.6 gal | 65.9 gal |
| 20 gpm | 58 gal | 67.7 gal | 77.5 gal |
Minimum total tank volume for a one-minute run, precharge 2 psi below cut-in. Usable share by Boyle's law; see the methodology.
The rule in one line
Tank volume = pump gpm × minimum run time ÷ usable share. The usable share between cut-in and cut-out, with the precharge set 2 psi below cut-in, is (precharge + 14.7) × (1 ÷ (cut-in + 14.7) − 1 ÷ (cut-out + 14.7)). At 40/60 that is 25.8%, so a 10 gpm pump needs about 38.8 gallons of tank. The sizing guide walks through it.
Usable share by switch setting
| Switch | Precharge | Usable share | Goulds Table 2 factor |
|---|---|---|---|
| 20/40 psi | 18 psi | 34.5% | 0.37 |
| 30/50 psi | 28 psi | 29.5% | 0.31 |
| 40/60 psi | 38 psi | 25.8% |
Goulds' factors assume the precharge equals the cut-in, which gives a slightly larger share; this site uses the 2 psi lower precharge AMTROL specifies and counts only water between the switch points. Either way the pattern is the same: the higher the switch setting, the smaller the share of the tank you can use, and the bigger the tank a given pump needs.
Why one minute
Franklin Electric's installation manual: "Run motors a minimum of one minute to dissipate heat build up from starting current." It also caps starts per day — 300 for single-phase motors of up to 0.75 hp, 100 for single-phase motors of 1 to 5.5 hp, 50 for single-phase motors of 7.5 to 30 hp. A tank sized for one minute is the floor; two minutes halves the starts and doubles the tank. For the pump itself, see well pump size by depth, and for precharge, the precharge guide.