Well Pressure Tank Calculator
Drawdown gallons and precharge from tank size.
What it does
Pressure tanks store a cushion of water so the well pump rests between uses. Only part of the tank drains before pressure falls to cut-in, and that usable slice is the drawdown this page figures.
The tool above takes tank size with the pressure switch setting. It returns usable drawdown with the acceptance factor and the air precharge beside it, in gallons or liters.
The defaults describe an 86 gal tank at 30/50 psi: factor 0.295 with 25.4 gal usable and 28 psi precharge. The metric mirror (326 L) gives 96.3 L with 1.93 bar precharge.
The Boyle law math with absolute pressures follows university extension well guidance, where drained tank pressure runs 2 psi under cut-in. National Ground Water Association pumping standards describe the same tank purpose and sizing practice.
How to use
- Pick imperial (gallons) or metric (liters).
- Enter total tank size.
- Pick the pressure switch setting.
- Read usable drawdown first.
- Read acceptance factor beside it.
- Set precharge with the tank empty.
Formula
- P1
- Precharge in absolute psi (gauge + 14.7).
- P2 / P3
- Cut-in and cut-out in absolute psi.
- factor
- Usable share of tank volume.
- 14.7
- Atmosphere in psi at sea level.
Air follows Boyle law, so higher cut-out squeezes the usable share: 40/60 draws less than 30/50 from the same tank. Check precharge only with the tank drained at zero gauge, since water inside fakes the reading.
Worked example
Example: 86 gal tank at 30/50
Defaults: imperial, 86 gal, 30/50 psi switch.
- Absolute: 42.7, 44.7, 64.7 psi.
- Factor: 42.7/44.7 - 42.7/64.7 = 0.295.
- Drawdown: 86 x 0.295 = 25.4 gal.
- Precharge: 30 - 2 = 28 psi.
- Tool shows 25.4 gal, 0.295 acceptance, 28 psi precharge.
- Metric mirror gives 96.3 L with 1.93 bar precharge.
Reference table
| Switch | Factor | Usable |
|---|---|---|
| 20/40 | 0.345 | 29.6 gal |
| 30/50 | 0.295 | 25.4 gal |
| 40/60 | 0.258 | 22.2 gal |
| 50/70 | 0.229 | 19.7 gal |
Limits
- Sea level 14.7 psi assumed throughout.
- Waterlogged bladders break the math.
- Altitude and heat shift air charge.
- Pump flow and well yield sit outside.
Common mistakes
- Counting full tank size as usable.
- Checking precharge with water inside.
- Raising pressure and expecting more water.
- Ignoring a short cycling pump.
- Skipping the 2 psi precharge rule.
Try it yourself
Quick checks. Answers sit tucked under each question.
86 gal at 40/60?
Factor 0.258 with 22.2 gal usable.
86 gal at 20/40?
Factor 0.345 with 29.6 gal usable.
Precharge rule?
2 psi under cut-in, tank empty at zero gauge.
Questions
What is drawdown?
Water a tank delivers between cut-out and cut-in pressure.
Why not the full tank?
Air charge holds pressure; only the acceptance share drains usable.
Where is precharge set?
2 psi under cut-in with the tank drained at zero gauge.
Higher pressure, more water?
No, higher settings shrink drawdown on the same tank.
Pump short cycling?
Usual suspects are wrong precharge or a failed bladder.
Bigger tank better?
Longer rests and fewer starts, up to space and budget.