Open the Check my tank tab and enter your tank size to see its drawdown, run time and pump starts per 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
What is the drawdown of a 20 gallon pressure tank?
About 5.9 gallons at 30/50 psi and 5.2 gallons at 40/60 psi with the precharge 2 psi below cut-in.
Why is drawdown so much less than the tank size?
The tank also holds the compressed air that pushes water out, so only roughly a quarter to a third of it is usable water.
Does a higher switch setting change drawdown?
Yes. A 40/60 switch gives less drawdown than 30/50 from the same tank, because air compresses less at higher pressure.
Is PressureTankCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.