How it works
A pressure tank holds a cushion of compressed air. As the pump fills the tank, the air compresses from the precharge to the cut-out pressure. The water you can draw out before the pump restarts is only the difference between how much space the air takes at cut-in and at cut-out. A "20 gallon" tank does not give you 20 gallons.
Boyle's law does the work: pressure × volume stays constant for the trapped air. All pressures must be absolute (gauge plus 14.7 psi). Divide drawdown by your pump flow and demand to see how long each run lasts and how often the pump starts.
Worked example
A 20-gallon tank on a 30/50 pressure switch with a 28 psi precharge (published examples list 5.9 gallons of drawdown), on a 10 gpm pump with 3 gpm of demand. A 20-gallon tank is small for a 10 gpm pump, and the short run time shows why:
- Absolute pressures: precharge 42.7, cut-in 44.7, cut-out 64.7 psi
- Acceptance factor = 0.9553 − 0.66 = 0.2953 (precharge ÷ cut-in, minus precharge ÷ cut-out)
- Drawdown = 20 gal × 0.2953 = 5.9 gal
- Run time = 5.9 gal ÷ (10 − 3 gpm) = 0.8 min
| Input | Value |
|---|---|
| Total tank volume | 20 gal |
| Pump cut-in pressure | 30 psi |
| Pump cut-out pressure | 50 psi |
| Tank air precharge (set with the tank empty) | 28 psi |
| Pump flow | 10 gpm |
| Average water demand while the pump runs | 3 gpm |
| Result | Value |
|---|---|
| Drawdown (usable water) | 5.9 gal |
| Acceptance factor | 29.5 % |
| Pump run time per cycle at that demand | 0.8 min |
| Pump starts per hour at that demand | 21.3 starts/hr |
Assumptions and limits
- Ideal gas at constant temperature. Real tanks lose a little to air absorption and temperature swings.
- Total tank volume is the full shell volume. Manufacturers' "drawdown" ratings assume specific pressure settings.
- Uses a constant water demand while the pump runs. Real households vary.
- Pump minimum run time and duty cycle limits come from the pump manufacturer.
Common questions
How much water does a 20 gallon tank hold usable?
At a 30/50 setting with a 28 psi precharge it delivers about 5.9 gallons; at 40/60 with 38 psi precharge, about 5.2 gallons. Wider pressure ranges and lower settings give more.
What should the precharge be?
Two psi below the pump cut-in pressure, checked with the tank drained of water and the pump off: 18 psi for a 20/40 switch, 28 for 30/50, 38 for 40/60.
Why does my pump short cycle?
Usually a waterlogged tank (lost air charge), a precharge set wrong, or a tank too small for the pump. Check the precharge with a tire gauge first.
Is a bigger tank always better?
A bigger tank gives longer run times and fewer starts, which is easier on the pump, but it costs more and needs the right precharge to deliver its drawdown.
Sources
- Boyle's law. Acceptance factor = P₁/P₂ − P₁/P₃ with absolute pressures, as published by pressure tank manufacturers and the Water Systems Council.
- Published check values: a 20-gallon tank drawdown is 5.9 gal at 30/50 with 28 psi precharge and 5.2 gal at 40/60 with 38 psi precharge.
- Read them: How to calculate and control pressure tank drawdown (The Driller)
Updated 2026-09-30