Check it with a tire gauge at the air valve with the pump off and a tap open until the water stops.
How the pressure tank calculator works
PressureTankCalc treats the air in the tank as an ideal gas at constant temperature (Boyle’s law). With pressures converted to absolute by adding 14.7 psi, the share of the tank that is usable water between cut-out and cut-in is precharge × (1 ÷ cut-in − 1 ÷ cut-out). The precharge defaults to 2 psi below cut-in, the setting tank makers recommend. The drawdown you need is the pump flow × the minimum run time, 1 minute by default; dividing it by the drawdown factor gives the total tank volume, rounded up to a common size. Pump starts per hour peak when household use is half the pump flow: 60 × GPM ÷ (4 × drawdown).
Drawdown by tank size and switch setting (gallons)
| Tank | 20/40 | 30/50 | 40/60 | 50/70 |
|---|---|---|---|---|
| 20 gal | 6.9 | 5.9 | 5.2 | 4.6 |
| 32 gal | 11 | 9.4 | 8.3 | 7.3 |
| 44 gal | 15.2 | 13 | 11.3 | 10.1 |
| 62 gal | 21.4 | 18.3 | 16 | 14.2 |
| 86 gal | 29.6 | 25.4 | 22.2 | 19.7 |
| 119 gal | 41 | 35.1 | 30.7 | 27.2 |
Drawdown factor by switch setting
| Switch | Precharge | Drawdown factor | Tank per GPM (1 min run) |
|---|---|---|---|
| 20/40 psi | 18 psi | 0.345 | 2.9 gal |
| 30/50 psi | 28 psi | 0.295 | 3.4 gal |
| 40/60 psi | 38 psi | 0.258 | 3.9 gal |
| 50/70 psi | 48 psi | 0.229 | 4.4 gal |
Makers’ tables differ slightly. Follow the pump and tank instructions, and have a well contractor check a pump that short cycles. Results are estimates.
Frequently asked questions
What should my pressure tank precharge be?
2 psi below the cut-in pressure of the pressure switch, measured with the tank drained and the pump off.
What happens if the precharge is too low?
The tank holds less usable water, so the pump starts more often. Very low or zero precharge usually means a failed bladder.
How often should I check the precharge?
Once a year is a good habit, and any time the pump starts cycling quickly.
Is PressureTankCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.