Well Pressure Tank Size Calculator (Drawdown and Pump Run Time)
What size well pressure tank you need: the drawdown for your pump's flow and minimum run time, the minimum total tank volume at your pressure-switch setting, and the smallest Amtrol Well-X-Trol model that delivers it.
Formula
Required drawdown = pump gpm × run minutes. Acceptance factor = (cut-out − cut-in) ÷ (cut-out + 14.7). Minimum total tank volume = drawdown ÷ acceptance factor. A tank's drawdown = volume × acceptance factor (capped at its maximum acceptance factor). Run time = drawdown ÷ gpm.
Worked example
Inputs: Pump flow rate = 10 gpm; Minimum pump run time = 1 minute (pumps up to 3/4 hp); Pressure switch cut-in = 40 psi; Pressure switch cut-out = 60 psi; Your tank's total volume (optional) = 0 gal.
| Required drawdown gal | 10 |
|---|---|
| Acceptance factor | 0.268 |
| Minimum total tank volume gal | 37.4 |
| Smallest well x trol model | WX-250 (44 gal, 11.8 gal drawdown) |
| Your tank drawdown gal | — |
| Your tank pump run minutes | — |
| Precharge psi | 38 |
Drawdown = tank volume × (cut-out − cut-in) ÷ (cut-out + 14.7), from Boyle's law with the air precharge at the cut-in pressure; this reproduces Amtrol's published drawdowns. Amtrol sizes for about 1 minute of pump run time for pumps up to 3/4 hp and about 2 minutes for larger pumps, and says to set the precharge 2 psi below cut-in. Bladder tanks with a low maximum acceptance factor are capped at it.
FAQ
What size pressure tank do I need for a 10 gpm pump?
At 40/60 psi with a 1-minute run time you need 10 gallons of drawdown. The acceptance factor is 20 ÷ 74.7 = 0.268, so the tank must hold at least 37.4 gallons total; the Well-X-Trol WX-250 (44 gal, 11.8 gal drawdown) is the smallest that does.
What is drawdown?
The water a tank delivers between the pump shutting off and turning back on. A bigger tank gives more drawdown, so the pump starts less often, which Amtrol says saves energy and extends pump life.
Why does a 44 gallon tank only deliver about 12 gallons?
The air charge takes up most of the tank. Between 40 and 60 psi it compresses by 20 ÷ 74.7 of its volume (absolute pressure), so 44 × 0.268 = 11.8 gallons. Amtrol lists 13.6 gallons at 30/50 and 10.4 at 50/70 for the same tank.
What should the tank precharge be?
Amtrol says 2 psi below the pressure switch cut-in: 28 psi for 30/50, 38 psi for 40/60 and 48 psi for 50/70. Check it with the tank empty of water.
1 minute or 2 minutes of run time?
Amtrol's sizing chart uses about 1 minute for pumps up to 3/4 hp and about 2 minutes for larger pumps. Check your pump maker's minimum run time as well.
Sources
- AMTROL Well-X-Trol specifications and Quick Sizing Chart (PDF): tank volume, maximum acceptance factor and drawdown at 30/50, 40/60 and 50/70 psig; ESP I ≈ 1 minute run time for pumps up to 3/4 hp, ESP II ≈ 2 minutes for larger pumps (copy hosted by InspectAPedia; amtrol.com blocks automated fetches)
- AMTROL Well Tank support FAQ: set the precharge 2 psi below the pressure-switch cut-in (e.g. 38 psi for 40/60)
Related
- Water Softener Size Calculator (Grains per Day and Capacity)
- Tankless Water Heater Size Calculator (GPM, Temperature Rise, BTU & kW)
Embed this calculator
Free to embed on your site; please keep the credit link.
Last reviewed 2026-10-30.