Use the calculator above for any other size, switch setting or precharge.
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
How much water does a 44 gallon pressure tank hold?
About 13 gallons of drawdown at 30/50 psi and 11.3 gallons at 40/60 psi.
How much drawdown does an 86 gallon tank have?
About 25 gallons at 30/50 psi and 22 gallons at 40/60 psi.
Is tank size the total or usable volume?
Model names usually refer to total volume. Check the drawdown in the maker’s chart, or use this calculator.
Is PressureTankCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.