A 44 gallon tank at 40/60 psi with a 10 GPM pump runs about 1.1 minutes per cycle and starts at most about 13 times an hour.
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 many times an hour should a well pump start?
Many motor makers allow up to about 20 starts an hour for small pumps and fewer for large ones; fewer starts mean longer life.
Why does my well pump turn on and off quickly?
Usually a waterlogged tank or failed bladder, low precharge or a tank too small for the pump. Check the precharge first.
What is a cycle stop valve?
A valve that throttles the pump to match demand so it runs continuously during long use, which allows a small tank.
Is PressureTankCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.