Well Pressure Tank Size Calculator
How much drawdown you need, the total tank volume that gives it, and the smallest Amtrol Well-X-Trol model that meets it.
What size pressure tank do I need?
Smallest single Amtrol Well-X-Trol tank whose drawdown meets flow × run time at 40/60 psi, from the drawdown column of Amtrol's spec card.
| Pump flow | Run time | Drawdown needed | Smallest WX tank (40/60) |
|---|---|---|---|
| 5 gpm | 1 min | 5 gal | WX-202 (5.4 gal) |
| 7 gpm | 1 min | 7 gal | WX-202XL (7.0 gal) |
| 10 gpm | 1 min | 10 gal | WX-250 (11.8 gal) |
| 12 gpm | 1 min | 12 gal | WX-251 (16.6 gal) |
| 15 gpm | 1 min | 15 gal | WX-251 (16.6 gal) |
| 20 gpm | 1 min | 20 gal | WX-255 (21.7 gal) |
| 5 gpm | 2 min | 10 gal | WX-250 (11.8 gal) |
| 7 gpm | 2 min | 14 gal | WX-251 (16.6 gal) |
| 10 gpm | 2 min | 20 gal | WX-255 (21.7 gal) |
Worked example
A 10 gpm pump up to 3/4 hp needs about 1 minute of run time, so 10 gallons of drawdown. At 40/60 psi the acceptance factor is 1 − (40+14.7)/(60+14.7) = 0.268, so the total tank volume is 10 ÷ 0.268 ≈ 37 gallons. On Amtrol's card the WX-205 gives 9.1 gal at 40/60 and the WX-250 gives 11.8, so the WX-250 is the smallest single tank that fits. That matches Amtrol's quick sizing table.
How it works
Drawdown is the water the tank delivers between the pump shutting off and turning back on. Amtrol sizes tanks so the pump runs at least about 1 minute (pumps up to 3/4 hp) or 2 minutes (larger pumps) per cycle, which means drawdown has to equal flow rate times run time. Total tank volume is drawdown divided by the acceptance factor, worked out from absolute pressures (gauge psi + 14.7). The precharge is set to the cut-in pressure, which is how Amtrol's drawdown figures are calculated.
Questions
What size pressure tank do I need for a 10 gpm pump?
With a 1-minute run time at 40/60 psi, Amtrol's quick sizing table lists the WX-250 (44 gallons total, 11.8 gallons drawdown).
Why is drawdown smaller than the tank size?
Only part of the tank holds water between cut-in and cut-out. At 40/60 psi that's about 27% of total volume, based on Amtrol's acceptance factor formula.
How do I estimate my flow rate?
The Water Systems Council gives three methods: 1 gpm per fixture, 3 gpm per fixture used at the same time, or 4 gpm per bathroom.
Sources
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