For pumps above 1.5 HP, enter 2 minutes as the minimum run time, as many motor makers recommend.
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 size pressure tank for a 10 GPM pump?
A 44 gallon tank at either 30/50 or 40/60 psi for a 1 minute run (about 34 and 39 gallons needed).
What size pressure tank for a 1 HP pump?
It depends on the flow: a 1 HP pump delivering 12–15 GPM needs a 62 gallon tank at 40/60 psi.
Can I use two smaller tanks instead of one large one?
Yes. Tanks piped together add their drawdowns, which also helps when space is tight.
Is PressureTankCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.