Size a well pressure tank by drawdown: pump flow in gallons per minute times the minimum run time. A 10 GPM pump needs about 10 gallons of drawdown for a 1-minute run, which at a 40/60 setting means roughly a 44-gallon tank, or 81–86 gallons for a 2-minute run. The calculator below does the math for your pump and switch setting.
How do you size a well pressure tank?
Multiply the pump's flow rate by the minimum run time to get the drawdown you need, then divide by the drawdown factor for your switch setting to get the total tank volume. Franklin Electric sets a 1-minute minimum run; Goulds calls 1 minute adequate and 2 minutes recommended; Amtrol's sizing chart uses about 2 minutes for pumps larger than ¾ hp.
The drawdown factor comes from Boyle's law: the air in the tank compresses as water enters. With the air pre-charge set 2 psi below cut-in, as tank and switch makers specify, the share of a tank you can actually use is:
| Switch setting | Pre-charge | Usable share of tank (drawdown factor) | Tank volume per gallon of drawdown |
|---|---|---|---|
| 20/40 psi | 18 psi | 34.5% | 2.9 gal |
| 30/50 psi | 28 psi | 29.5% | 3.4 gal |
| 40/60 psi | 38 psi | 25.8% | 3.9 gal |
| 50/70 psi | 48 psi | 22.9% | 4.4 gal |
Manufacturer drawdown tables run slightly higher than these figures because they assume the pre-charge equals the cut-in pressure, so the calculator uses the more conservative number. Higher pressure settings shrink the usable share, which is why a 40/60 system needs a bigger tank than a 30/50 one for the same pump.
Choosing 50/70 only makes sense if your pump is rated to build 70 psi at your well depth (otherwise it never shuts off) and your tank is rated well above 70 psi. Raise the pre-charge to 48 psi, and leave switch adjustment to a pro if you aren't comfortable working near 240-volt terminals.
Common tank sizes and their real drawdown
Nominal size is the total volume, not the water you get. Amtrol's published drawdowns for its Well-X-Trol line show the gap:
| Amtrol model | Total volume | Drawdown at 30/50 | Drawdown at 40/60 |
|---|---|---|---|
| WX-202 | 20 gal | 6.2 gal | 5.4 gal |
| WX-203 | 32 gal | 9.9 gal | 8.6 gal |
| WX-205 | 34 gal | 10.5 gal | 9.1 gal |
| WX-250 | 44 gal | 13.6 gal | 11.8 gal |
| WX-251 | 62 gal | 19.2 gal | 16.6 gal |
| WX-255 | 81 gal | 25.0 gal | 21.7 gal |
| WX-302 | 86 gal | 26.6 gal | 23.0 gal |
| WX-350 | 119 gal | 36.8 gal | 31.9 gal |
Other brands publish similar tables, so compare drawdown at your switch setting, not the number on the box. Two tanks can also be plumbed together: their drawdowns add up.
Which rules of thumb hold up?
- "1 gallon of drawdown per GPM": correct as a floor. It's the 1-minute rule.
- "Tank volume is about 4 times the drawdown": close at 40/60, where the factor is about 26%.
- "Tank size equals 2 times the GPM": too small. At 40/60 it gives about a 30-second run, which is short-cycling territory.
- "10% of pump output": that's Franklin's minimum tank size for constant-pressure (variable-speed) systems, which need much less tank. Don't use it for a standard pressure-switch system.
What does a bigger tank cost?
Installed, a standard 20–44-gallon tank typically runs $300–$1,600 and an 80–120-gallon tank $850–$2,500, based on Angi and HomeGuide estimates. That is still well below a $1,200–$3,500 pump replacement, which is what an undersized tank eventually costs you. To check the demand side of the system, run the well capacity calculator.
Frequently Asked Questions
What size pressure tank do I need for a 10 GPM pump?
At a 40/60 switch setting, a 10 GPM pump needs about 10 gallons of drawdown for a 1-minute run, which a 44-gallon tank like Amtrol's WX-250 (11.8 gallons of drawdown) provides. For the 2-minute run Amtrol's sizing chart uses for pumps larger than ¾ hp, you need about 20 gallons of drawdown, which takes an 81–86-gallon tank.
What is pressure tank drawdown?
Drawdown is the water a tank delivers between the pump shutting off and turning back on. It is only a fraction of the tank's nominal size because most of the tank holds compressed air: about 26% at a 40/60 setting and about 30% at 30/50, with the air pre-charge set 2 psi below cut-in.
How long should a well pump run each cycle?
At least one minute. Franklin Electric's motor manual says to run motors a minimum of one minute so they can shed the heat from starting, and Goulds calls a one-minute cycle adequate and two minutes recommended. Amtrol's sizing chart uses about two minutes for pumps larger than ¾ hp.
Is a bigger pressure tank always better?
Mostly, up to the space and budget you have. More drawdown means fewer pump starts per day, which extends pump life. The trade-offs are cost, floor space and weight, and water sitting longer in the tank. There's little benefit past the size that gives your pump a 2-minute run.
Sources
- Amtrol — Well-X-Trol Specifications and ESP Quick Sizing Table
- Goulds Water Technology (Xylem) — HYDRO-PRO Tank Sizing Guide
- Franklin Electric — AIM Manual for Submersible Motors (60 Hz)
- Angi — How Much Does a Well Tank Replacement Cost?
We check every figure on this page against these sources. Rules and prices change, so confirm local requirements with your county or state office. Spot an error? Tell us.