Sizing a well pressure tank means matching its drawdown to your pump’s flow rate and pressure switch, not buying by the tank’s total gallon label.
Most people pick a replacement by copying whatever gallon size the old tank had. That approach fails because the number on the nameplate is total volume, and only a fraction of it is usable water. The correct way to size a pressure tank for a well starts with two readings: the pump’s flow rate in gallons per minute (GPM) and the pressure switch’s cut-in and cut-out settings. Get those two numbers and the rest is a short calculation.
Why the Gallon Label Is Misleading
A pressure tank is part water, part compressed air. The air cushion is what pushes water to the house, and only the water released between the switch’s cut-in and cut-out pressures is “drawdown” — the usable amount before the pump restarts. In a standard bladder tank, that usable fraction is small. One 20-gallon tank, for example, may deliver only about 6 to 10 gallons of drawdown, depending on the pressure setting.
This is why sizing by the nameplate sends people wrong: two tanks with the same gallon rating can deliver different drawdown at different pressure settings, and the pressure you run changes the math completely.
How To Calculate Minimum Drawdown
Size the tank so the pump runs long enough to avoid rapid cycling, which wears out motors and pressure switches. Common industry guidance calls for a minimum run time of about 1 minute for motors under 2 HP, and 2 minutes for 2 HP and above. From there the formula is simple:
Pump flow rate (GPM) × desired run time (minutes) = required drawdown.
For a 10 GPM pump running 2 minutes, that means at least 20 gallons of usable drawdown. Sizing tables translate that into practical targets by flow range:
| Pump Flow Rate | Target Drawdown Per GPM |
|---|---|
| 0–10 GPM | About 1 gallon per GPM |
| 10–20 GPM | About 1.5 gallons per GPM |
| 20+ GPM | About 2 gallons per GPM |
Matching Pressure Settings To A Real Tank
Since pressure settings drive everything, the final step is checking the manufacturer’s drawdown chart for the exact tank and pressure range you plan to run. A 30/50 system and a 40/60 system at the same GPM can call for different tank sizes, and identical-looking tanks at the same pressure can vary. The sizing calculators from Amtrol and the guides published by Water Well Journal both make this same point: confirm the model against its chart before buying.
Once the tank arrives, set the air precharge to 2 psi below the switch’s cut-in pressure with the tank empty before startup. That precharge is what keeps the air cushion working as designed.
Common Sizing Mistakes To Avoid
The biggest error is sizing by total gallons instead of drawdown — a “32-gallon” tank is not 32 gallons of water. The second is ignoring the cut-in/cut-out numbers when comparing tanks, and the third is choosing a tank too small for the flow rate, which causes rapid cycling and shortens pump life. A quick rule many installers use: target at least 1 gallon of drawdown per GPM of pump output as a minimum, then verify the tank’s actual drawdown at your pressure setting.
For low-flow systems, a 20-gallon tank can be the right minimum at 5 GPM and 30/50 psi. If that’s the size you need, our tested roundup of the best 20-gallon pressure tanks compares models that differ in real drawdown, so you can match one to your switch settings.
The takeaway: get the pump’s GPM, note the pressure switch settings, calculate drawdown, and check the manufacturer’s chart before you order. That sequence protects the pump and ends the guesswork.
FAQs
Is a bigger pressure tank always better?
Bigger is usually safer than too small, because more drawdown means longer pump run times and fewer cycles. But a tank that is drastically oversized for the well’s flow rate can mean stale water sitting longer and a higher purchase price, so matching drawdown to the pump’s GPM is the better target.
What happens if the pressure tank is too small?
A tank with too little drawdown makes the pump short-cycle, turning on and off rapidly as the pressure drops to cut-in almost immediately. That cycling heats the motor, wears out the pressure switch contacts, and can cut pump life significantly. Fixing it usually means stepping up to the next tank size at the same pressure setting.
Does the same tank deliver more water at 40/60 than at 30/50?
Yes. Drawdown is the water released between cut-in and cut-out, so the wider pressure swing and higher air compression at 40/60 psi lets the same tank deliver more usable water than it does at 30/50 psi. That is why the same pump can need a different tank depending on the switch settings.
References & Sources
- Water Well Journal. “Tank Selection By The Numbers.” Explains the drawdown formula, run-time targets, and sizing guidance by GPM.
- Westank. “Expansion Tank Sizing Calculator.” Interactive tool for sizing by system input; notes results depend on user settings.
- Amtrol. “Selection Tools.” Manufacturer sizing resources for well and pressure booster tanks.
