Pressure Tank Size Calculator
A 10 GPM pump running 1.5 minutes per cycle needs 15 gallons of drawdown. At a 40/60 PSI switch only about 27% of a tank's volume is usable water (Amtrol's Well-X-Trol spec card, Mar 2026, lists 11.8 gallons of drawdown for its 44-gallon tank), so that takes at least 56 gallons of tank, which our calculator rounds up to the next Amtrol size, 62 gallons. A pressure tank's job is to let your pump rest: once it starts, it should run at least one minute, ideally 1.5 to 2. That run time plus your pump's flow rate sets the drawdown you need, and drawdown (not the number on the tank) is what you're actually buying.
Enter three things below and get the nominal tank size that keeps your pump from short-cycling itself to death. Undersizing is the expensive mistake here — a too-small tank quietly wears out a $2,000 pump.
For example: a 10 GPM pump on a 40/60 switch with a 1-minute run time needs 10 gallons of drawdown (10 ÷ 0.268 ≈ 37 gallons of tank), so the calculator recommends a 44-gallon nominal tank, the same size Amtrol's own sizing table lists for that pump.
Pressure tank size chart by pump GPM
Drawdown = pump GPM × run time. Nominal tank size = drawdown ÷ the drawdown factor for your switch. At the recommended 1.5-minute run time:
| Pump flow | 30/50 PSI | 40/60 PSI | 50/70 PSI |
|---|---|---|---|
| 5 GPM | 26 gal | 32 gal | 32 gal |
| 8 GPM | 44 gal | 62 gal | 62 gal |
| 10 GPM | 62 gal | 62 gal | 81 gal |
| 12 GPM | 62 gal | 81 gal | 81 gal |
| 15 GPM | 81 gal | 86 gal | 119 gal |
| 20 GPM | 119 gal | 119 gal | Over 119 gal |
Calculated from drawdown factors derived from Amtrol's Well-X-Trol spec card (Mar 2026), rounded up to the next Well-X-Trol size. Over 119 gallons usually means two tanks or a constant-pressure system.
How to size a pressure tank
The calculation is the standard two-step tank-sizing method used across the industry. Step one, drawdown: multiply your pump's flow rate by the minimum run time you want per cycle (drawdown = GPM × minutes). A 10 GPM pump targeting a 1.5-minute run needs 15 gallons of usable water between pump starts. One minute is the accepted floor; 1.5–2 minutes measurably extends motor life because starting current — not running — is what wears a pump out.
Step two, converting drawdown to a nominal tank size: a diaphragm tank only yields part of its rated volume as usable water, and that share shrinks as system pressure rises. We use drawdown factors derived from Amtrol's Well-X-Trol published specifications (Mar 2026): 0.31 at a 30/50 switch, 0.268 at 40/60, and 0.236 at 50/70 — so required nominal size = drawdown ÷ factor. That 15-gallon drawdown at 40/60 means 15 ÷ 0.268 ≈ 56 gallons, which rounds up to the next nominal size: 62 gallons. We round up through Amtrol's Well-X-Trol lineup (20, 26, 32, 44, 62, 81, 86 and 119 gal) because between sizes, bigger only costs a little more while smaller costs pump life; needs beyond 119 gallons call for twin tanks or a constant-pressure system.
Real installs add details this calculator can't see: the tank's air precharge must be set 2 PSI below the switch cut-on, high-flow pumps may pair better with a variable-speed controller than a huge tank, and a short-cycling system may already have a damaged pump. Replacement tanks run $300–$700 installed (Angi, avg $500) — the independent local pros we connect you with will confirm the size against your actual pump before quoting.
Estimates only — independent local providers quote their own pricing. Data last reviewed 2026-10.
Sources
- Fresh Water Systems — How to Size a Well Pressure Tank
- Angi — Well Pressure Tank Replacement Cost (Sep 18, 2026) (third-party estimate)
- Amtrol Well-X-Trol Spec Card (Mar 2026)
Frequently Asked Questions
What size pressure tank do I need?
Multiply your pump's GPM by a minimum run time of 1–2 minutes to get required drawdown, then divide by the drawdown factor for your switch setting (0.31 at 30/50, 0.268 at 40/60, 0.236 at 50/70). A typical 10 GPM pump at 40/60 on a 1.5-minute run needs about 15 gallons of drawdown: 15 ÷ 0.268 ≈ 56 gallons, so a 62-gallon nominal tank.
Is a bigger pressure tank better?
Generally yes, within reason. A larger tank means longer run cycles and fewer starts, and starts are what kill pump motors. The tradeoffs are only price and floor space — oversizing doesn't hurt the system. When you're between standard sizes, go up.
Why does a 30-gallon tank only hold about 8 gallons of water?
Nominal size is the tank's total volume, but most of it holds the compressed-air cushion behind the diaphragm. Only 25–31% is usable water (drawdown), depending on your pressure switch setting — higher pressure ranges leave less room for water. That's why sizing works from drawdown, not the sticker number.
How much does a new pressure tank cost installed?
Most replacements run $300–$700 installed, with an average around $500 (Angi). Larger all-metal or commercial diaphragm tanks can reach $2,500. It's a same-day job. If the old tank failed by waterlogging, have the installer check the pump — short cycling may have already shortened its life.
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