How to Size a Well Pressure Tank
Sizing a well pressure tank comes down to one number: your pump’s gallons per minute (GPM). Get that right, and the rest follows a simple formula. Get it wrong, and you end up with a pump that short-cycles every few minutes, wearing out a $200+ motor years before it should fail.
This guide covers the quick sizing table, the draw-down math, and why spending an extra $50–$100 on a bigger tank is almost always worth it. For the full picture on your pressure switch and tank guide, see our hub page.
Quick sizing table
Start here. Match your pump’s GPM to the correct tank size, then go one size up.
| Pump GPM | Household Size | Minimum Tank | Recommended |
|---|---|---|---|
| 5 GPM | 1–2 people | 14 gallon | 20 gallon |
| 8 GPM | 2–3 people | 20 gallon | 32 gallon |
| 10 GPM | 3–4 people | 32 gallon | 44 gallon |
| 15 GPM | 4–5 people | 44 gallon | 62 gallon |
| 20 GPM | 5+ people | 62 gallon | 86 gallon |
Always buy one size up from the minimum. We recommend going two sizes up if you have any plans for future expansion, the cost difference is $50–$100 now versus draining and replumbing the system later. A larger tank costs less in the long run by reducing pump cycling. See why in the next section.
For more guidance on which tank model to buy, see our pressure tank buying guide.
Video guide
Video: “What Size Well Pump Do You Need? How to install a Pump Correctly” by H2O Mechanic
The draw-down formula
Every pressure tank holds more water than you can actually use. The usable portion, called draw-down, is what matters for sizing.
A well pressure tank’s draw-down is roughly 25% of total tank volume at a standard 30/50 PSI setting. That means a 32-gallon tank gives you about 8 gallons of usable water before the pump kicks on.
The formula:
- Find your pump’s GPM (covered in the next section)
- Multiply GPM × 1 minute = minimum draw-down required
- Divide draw-down by 0.25 (for 30/50 PSI) = minimum tank volume
Example: A 10 GPM pump needs at least 10 gallons of draw-down. Divide 10 ÷ 0.25 = 40 gallons of tank volume. The closest standard size is a 44-gallon tank.
To calculate minimum tank size, multiply your pump’s GPM by 1 minute of run time to get required draw-down, then divide by 0.25 (for 30/50 PSI) to get total tank volume needed.
Why bigger is better
Every time your pump starts, it draws a surge of current through the motor windings. This startup stress is the primary cause of motor failure.
A pump motor rated for 300,000 starts will last 15+ years with a properly sized tank that limits cycling. Run that same pump against an undersized tank (one that forces it to cycle more than 6 times per hour) and motor life drops to 3-5 years.
The math makes sense: pressure tanks start at $200 (per Family Handyman’s cost data), while pump replacement runs $300–$2,000 including labor. Spending an extra $50–$100 to jump from a 20-gallon to a 32-gallon tank is straightforward economics.
The EPA well system maintenance{:target=“_blank”} guidelines recommend annual system checks, including verifying that your pressure tank is correctly sized for your pump’s output.
Draw-down by pressure setting
Your pressure switch setting changes how much usable water you get from a given tank. Higher pressure settings compress the air charge more, which leaves less room for draw-down.
| Pressure Setting | Approximate Draw-Down (% of tank volume) |
|---|---|
| 20/40 PSI | 32% |
| 30/50 PSI | 25% |
| 40/60 PSI | 20% |
If you run a 40/60 system (common in multi-story homes), you need a larger tank than a 30/50 system with the same pump GPM. A homeowner with a 10 GPM pump on 40/60 settings needs at least 50 gallons (10 ÷ 0.20 = 50), not 40 gallons.
Check your pressure switch settings before finalizing your tank size.
How to find your pump’s GPM
Check the pump nameplate first. Most residential submersible pumps list rated flow in GPM on the label near the motor housing. Typical residential submersibles run 5–15 GPM.
If the nameplate isn’t accessible (buried submersibles often aren’t), run a bucket test:
- Connect a hose to a hose bib downstream of the pump
- Disconnect the pressure tank from the system temporarily, or just run water with the tank isolated
- Fill a 5-gallon bucket and time it with a stopwatch
- GPM = 5 ÷ fill time in minutes
Example: If the bucket fills in 45 seconds (0.75 minutes), GPM = 5 ÷ 0.75 = 6.7 GPM. Round up to 8 GPM for sizing purposes.
The Well-X-Trol sizing specifications{:target=“_blank”} from Amtrol (one of the largest tank manufacturers) also include a flow rate chart for cross-referencing pump nameplate data.
Common sizing mistakes
We see the same errors repeatedly in forums and service calls:
- Matching old tank size: many homes were built with undersized tanks that caused early pump failures. Don’t assume the original was correctly sized.
- Sizing by household, not GPM: household size is a proxy, not the actual variable. Always calculate from pump GPM.
- Choosing the cheapest option: the price difference between minimum and recommended tank size is $50-$100. The pump you protect is worth $300-$1,500.
- Ignoring future demand: a new bathroom adds 1-2 GPM peak demand; an irrigation zone adds 3-5 GPM. If expansion is possible, size up.
For help diagnosing whether your current setup has pressure tank problems, see our troubleshooting guide.
Tank size vs physical size
Before ordering, measure your pressure tank installation space. Tank footprints grow significantly with volume.
| Model Volume | Approximate Dimensions | Weight (empty) |
|---|---|---|
| 20 gallon | 16” × 30” | 30 lbs |
| 32 gallon | 18” × 36” | 40 lbs |
| 44 gallon | 22” × 43” | 55 lbs |
| 62 gallon | 22” × 50” | 70 lbs |
| 86 gallon | 24” × 59” | 90 lbs |
A 44-gallon tank is 43 inches tall, too tall for crawl spaces with standard 36-inch clearance. In those situations, some homeowners install two smaller tanks in parallel rather than one large one.
For an in-depth look at well system maintenance when things go wrong, see our well pump repair overview.
FAQ
What size pressure tank do I need for a 3/4 HP pump?
Tank size depends on GPM output, not motor horsepower. A 3/4 HP submersible pump typically produces 10–12 GPM, which calls for a minimum 32-gallon tank and ideally a 44-gallon tank for a standard 30/50 PSI system. Verify your pump’s actual GPM with the nameplate or a bucket test before purchasing.
Can a pressure tank be too big?
In practice, no. A larger tank simply means fewer pump starts per day and longer motor life. The only downside is physical space. A 62-gallon tank in a small utility room can be awkward. There is no pressure performance penalty for oversizing.
How do I know if my current tank is undersized?
If your pump cycles more than 6 times per hour during normal household use, the tank is undersized (or waterlogged). To check: count pump starts over a 10-minute period while running a faucet at moderate flow. More than 1 start per minute points to a tank problem.
Does pressure switch setting affect tank size?
Yes. At 30/50 PSI, draw-down is about 25% of tank volume. At 40/60 PSI, draw-down drops to roughly 20%. A 10 GPM pump on a 40/60 system needs a 50-gallon tank minimum versus a 40-gallon tank on 30/50. Always factor in your actual switch settings when running the draw-down formula.