What Size Pressure Tank Do I Need?
The right well pressure tank size is the one that holds enough usable water for your pump to run at least a minute each cycle, or two minutes for pumps over 3/4 hp, which is the run time Amtrol sizes its tanks around. In practice that means sizing from your pump's flow rate in gallons per minute, not from the number of people in the house or the gallons printed on the old tank.
Drawdown is the number that matters
A pressure tank is mostly air. The amount of water it actually pushes out between pump cycles is called drawdown, and it's far smaller than the tank's listed capacity. It also shrinks as you raise the pressure switch setting. Amtrol's figures for some common Well-X-Trol models:
| Model | Tank volume | Drawdown at 30/50 | Drawdown at 40/60 |
|---|---|---|---|
| WX-202 | 20 gallons | 6.2 gallons | 5.4 gallons |
| WX-203 | 32 gallons | 9.9 gallons | 8.6 gallons |
| WX-250 | 44 gallons | 13.6 gallons | 11.8 gallons |
| WX-251 | 62 gallons | 19.2 gallons | 16.6 gallons |
| WX-255 | 81 gallons | 25.0 gallons | 21.7 gallons |
| WX-350 | 119 gallons | 36.8 gallons | 31.9 gallons |
So a "44-gallon" tank on a 40/60 switch delivers about 12 gallons before the pump starts. Amtrol adds that drawdown shifts with temperature and system pressure.
How to size it
The math is simple: your pump's flow rate times the minimum run time is the drawdown you need. Amtrol's sizing guide uses exactly that, pump gallons per minute times run time in minutes.
- Find your pump's flow rate. It's on the pump paperwork, or your well contractor can measure it. A rough home check is to time how long it takes to fill a bucket of known size while the pump is running.
- Pick the run time. Amtrol's card uses 1 minute for pumps up to 3/4 hp and 2 minutes for pumps over 3/4 hp.
- Multiply. Using that gpm-times-minutes math, a 10 gpm pump under 3/4 hp needs about 10 gallons of drawdown.
- Find a tank with at least that drawdown at your switch setting. From the table, a WX-250 covers it on either a 30/50 or a 40/60 switch; the WX-203's 9.9 gallons at 30/50 falls just short.
Amtrol's quick sizing chart gives the same picks:
| Pump flow | Up to 3/4 hp, 40/60 switch | Over 3/4 hp, 40/60 switch |
|---|---|---|
| 5 gpm | WX-202 | WX-250 |
| 7 gpm | WX-203 | WX-251 |
| 10 gpm | WX-250 | WX-255 |
| 12 gpm | WX-251 | WX-350 |
| 15 gpm | WX-251 | WX-350 |
Other brands publish their own drawdown charts. Use the chart from the maker of the tank you're buying.
The quick household rule
If you don't know your pump's flow, HomeGuide's rough guide is 20 to 40 gallons for 1 to 2 people, 40 to 60 gallons for 3 to 4 people, and 80 gallons or more for larger households. That's fine for a ballpark. But a family of five on a big submersible pump can still short cycle a mid-size tank, which is why the pump-based method above is the one to trust.
Is bigger better?
Usually, within reason. Amtrol says the larger the tank, the greater the drawdown and the less the pump needs to run. Fewer starts means less wear on the pump motor and the pressure switch contacts, and Amtrol notes that rapid cycling can lead to pump failure. Oversizing costs more up front and takes more floor space, but it rarely causes problems. Undersizing is what burns out pumps.
Two limits: a bigger tank won't fix a pump that can't keep up with demand, and it won't fix low pressure at the tap. Those are pump and switch problems.
What to do next
- Pricing a new tank: see well pressure tank replacement cost.
- Doing the swap: how to replace a well pressure tank.
- Pump already short cycling with a decent-size tank? Check the air charge first, then pressure tank troubleshooting.
FAQ
What size pressure tank do I need for a family of 4?
HomeGuide's household guide puts 3 to 4 people at 40 to 60 gallons. Check it against your pump, though. If your pump is over 3/4 hp, Amtrol sizes for a 2-minute run time, which can call for a bigger tank.
What size pressure tank for a 1 hp pump?
It depends on the pump's flow, not just its horsepower. A 1 hp pump falls in Amtrol's over-3/4-hp column, so you size for a 2-minute run. At 10 gpm on a 40/60 switch, Amtrol's chart picks the 81-gallon WX-255.
Is a 20-gallon pressure tank big enough for a house?
For a small pump and one or two people, it can be. Amtrol matches its 20-gallon WX-202 to pumps of about 5 gpm up to 3/4 hp. For anything bigger, it's on the small side.
Can I use two smaller tanks instead of one big one?
Yes. Amtrol's quick sizing chart lists some large-pump picks with a (2) or (3) after the model, which reads as two or three tanks. Their drawdown adds up, and they can fit where one big tank won't.
Sources
- AMTROLWell-X-Trol Quick Sizing Guide (opens in a new tab)
- AMTROLWell-X-Trol Homeowner Handout (opens in a new tab)
- HomeGuideHow Much Does It Cost to Replace a Well Pressure Tank? (opens in a new tab)
- AMTROLWell-X-Trol Residential Pre-Pressurized Well Tanks Installation and Operation Instructions (opens in a new tab)
- AMTROLWell-X-Trol Sizing Guide (opens in a new tab)