Pressure-tank drawdown / acceptance factors
A pressure tank only delivers a fraction of its total volume between the pump’s cut-in and cut-out pressures — the drawdown. The acceptance factor below turns a required drawdown into a total tank size (tank = drawdown ÷ factor). A wider pressure band (e.g. 40/60) gives more usable drawdown per gallon of tank. These are labeled conventions — check the tank’s own rating. Use them in the pressure-tank sizing tool.
| Cut-in / cut-out (psi) | Acceptance factor | Tank for 15 gal drawdown |
|---|---|---|
| 20-40 | ~0.20 | 75 gal total |
| 30-50 | ~0.30 | 50 gal total |
| 40-60 | ~0.40 | 38 gal total |
Labeled conventions — check the pressure tank’s own drawdown rating. Total tank size = required drawdown ÷ acceptance factor; a wider band (40/60) gives more usable drawdown per gallon of tank.