What Size Pressure Tank Do I Need? Simple Sizing Guide
Pick a pressure tank that's too small and your pump short cycles and wears out early. Pick one that's far too big and you've paid for steel you don't need. The right size comes down to one number: how much water your pump puts out per minute. This guide shows the simple math installers use.
Tank size vs drawdown
A tank's gallon rating (20, 33.5, 81 gallons) is its total volume. What matters is drawdown: the water the tank delivers between pump shut-off and pump restart. Drawdown is only a fraction of the total, and it changes with your pressure switch setting. A tank delivers more drawdown at 20/40 PSI than at 40/60 PSI. Always check the maker's drawdown chart for your switch setting.
The 1-minute rule
Most pump makers want the motor to run at least one minute per start (some ask for two minutes on larger motors). So the tank's drawdown should be at least equal to your pump's flow rate in gallons per minute (GPM) times the minimum run time.
| Pump flow | Minimum drawdown (1 min run) | Typical tank size class* |
|---|---|---|
| 5–7 GPM | 5–7 gal | 20–35 gal |
| 8–10 GPM | 8–10 gal | 35–45 gal |
| 12–15 GPM | 12–15 gal | 60–85 gal |
| 20+ GPM | 20+ gal | Multiple tanks or a cycle stop valve |
*Rough guide at a 30/50 or 40/60 setting. Confirm with the tank's drawdown chart.
How to find your pump's GPM
- Check the pump label or model. For example, the Goulds J5S, J7S and J10S are 1/2, 3/4 and 1 HP shallow well jet pumps; the maker's performance curve shows GPM at your lift and pressure.
- Bucket test. With the tank empty and the pump running, time how long it takes to fill a 5-gallon bucket from a spigot near the tank. 5 ÷ seconds × 60 = GPM.
- Call us with the model number and we'll look up the curve for you.
Household demand matters too
Larger homes, irrigation zones, livestock and water treatment backwash all draw long, steady flows. If you regularly run long draws, size up so the pump runs longer and starts less. A rough household rule is 1 GPM per fixture (bathrooms, kitchen, laundry, outdoor spigots), but irrigation needs its own calculation.
Popular sizes
- Flexcon Challenger PC66FR, 20 gallon: smaller homes and lower-flow jet pumps.
- Flexcon Challenger PC122, 33.5 gallon: a common pick for typical 3-bedroom homes.
- Flexcon Challenger PC244FR, 81 gallon: higher-flow submersibles and bigger households.
See every option in pressure tanks.
Can't fit a bigger tank?
A CSV1A Cycle Stop Valve (up to 25 GPM) lets a smaller tank work by keeping the pump running steadily during long draws, so it doesn't cycle. Constant pressure controllers such as the Grundfos CU 301 for SQE pumps are another route.
Winter tip
If you're replacing a tank this winter, check where it will sit. A tank in an unheated space needs freeze protection, and its air pre-charge should be set (about 2 PSI under cut-in) with the tank empty. See our jet pump winterizing guide.
Worked example
Say a bucket test shows your jet pump delivers 9 GPM and your switch is set at 40/60. For a 1-minute minimum run, you need at least 9 gallons of drawdown at 40/60. At that setting, drawdown is often only about a quarter of a tank's total volume, so you'd look at a tank in roughly the 35–45 gallon class, then confirm with the maker's chart. If your household also runs a sprinkler zone or a water softener backwash, step up a size.
Where to put the tank
Install the tank close to the pressure switch, on a level base, in a spot that stays above freezing. Leave room to reach the air valve on top for yearly pre-charge checks. Larger tanks are heavy when full, so make sure the floor can carry the weight.
Sizing checklist
- Pump flow (GPM) at your pressure.
- Pressure switch setting.
- Minimum run time required by the pump maker.
- Biggest regular water draw in the home.
- Space and floor strength available.
FAQ
Is a bigger pressure tank always better?
Bigger means fewer starts and longer pump life, which is good. The trade-offs are cost, floor space and weight. Size to at least a 1-minute run time and go bigger if you use a lot of water.
What size pressure tank for a 1 HP pump?
It depends on the pump's GPM at your pressure, not the horsepower. Many 1 HP pumps deliver around 8–12 GPM, which typically points to a tank in the 35–85 gallon class. Send us the model and we'll confirm.
Can I add a second tank?
Yes. Two tanks piped together add their drawdowns. Set both to the same pre-charge.
What pre-charge should my tank have?
About 2 PSI below your pressure switch cut-in, set with the tank empty: 28 PSI for 30/50, 38 PSI for 40/60.
Need a hand? Call our pump team at (866) 202-8388 (Mon–Fri, 9 AM–5 PM ET) or email help@supremewatersales.com with your pump model, well depth and tank size. We will tell you what fits before you buy, no obligation.