How the calculator works
- Drawdown needed = pump GPM × minimum run time (default 1 minute, Franklin Electric).
- Drawdown factor (Boyle's law, gauge psi + 14.7) = Pprecharge × (1/Pcut-in − 1/Pcut-out). 40/60 with 38 psi precharge: 0.258.
- Total tank volume = drawdown needed ÷ factor.
Example: 10 GPM, 40/60, 1 minute → 10 gallons drawdown, 38.8 gallons total. Smallest Amtrol that clears it: WX-250 (44 gal), the same tank Amtrol's own card picks.
The three precharge conventions differ by a few percent: makers' printed drawdowns assume precharge equal to cut-in (4–5% more water), and Goulds' HydroPro table counts down to the precharge (about 10% more again). The default is the smallest.
Pressure tank size chart by GPM and switch setting
Total volume for 1 minute of run time, precharge 2 psi below cut-in, with the smallest Amtrol Well-X-Trol that meets it. Double for 2 minutes.
| Pump GPM | 20/40 | 30/50 | 40/60 | 50/70 |
|---|---|---|---|---|
| 5 | 14.5 gal WX-202 | 16.9 gal WX-202 | 19.4 gal WX-202 | 21.9 gal WX-202XL |
| 7 | 20.3 gal WX-202XL | 23.7 gal WX-202XL | 27.1 gal WX-203 | 30.6 gal WX-203 |
| 10 | 29.0 gal WX-203 | 33.9 gal WX-205 | 38.8 gal WX-250 | 43.7 gal WX-250 |
| 12 | 34.8 gal WX-250 | 40.6 gal WX-250 | 46.5 gal WX-251 | 52.4 gal WX-251 |
| 15 | 43.5 gal WX-250 | 50.8 gal WX-251 | 58.2 gal WX-251 | 65.6 gal WX-255 |
| 20 | 58.0 gal WX-251 | 67.7 gal WX-255 | 77.5 gal WX-255 | 87.4 gal WX-350 |
Drawdown factors by setting
| Setting | Precharge | 2 psi below cut-in | Precharge = cut-in | Goulds Table 2 | Goulds "extra 2 psi" |
|---|---|---|---|---|---|
| 20/40 | 18 psi | 0.345 | 0.366 | 0.37 | 0.402 |
| 30/50 | 28 psi | 0.295 | 0.309 | 0.31 | 0.340 |
| 40/60 | 38 psi | 0.258 | 0.268 | 0.27 | 0.295 |
| 50/70 | 48 psi | 0.229 | 0.236 | 0.24 | 0.260 |
Amtrol Well-X-Trol: volume and drawdown by setting
Printed values from Amtrol MC7025 (03/24). The 20/40 column is our calculation (volume × 0.366).
| Model | Total volume (gal) | 20/40 (calc) | 30/50 | 40/60 | 50/70 |
|---|---|---|---|---|---|
| WX-101 | 2.0 | 0.7 | 0.6 | 0.6 | 0.5 |
| WX-102 | 4.4 | 1.6 | 1.4 | 1.2 | 1.0 |
| WX-103 | 6.7 | 2.4 | 2.1 | 1.8 | 1.6 |
| WX-104 | 10.3 | 3.8 | 3.2 | 2.8 | 2.4 |
| WX-200 | 14.0 | 5.1 | 4.3 | 3.8 | 3.3 |
| WX-201 | 14.0 | 5.1 | 4.3 | 3.8 | 3.3 |
| WX-202 | 20.0 | 7.3 | 6.2 | 5.4 | 4.7 |
| WX-202XL | 26.0 | 9.5 | 8.0 | 7.0 | 6.1 |
| WX-203 | 32.0 | 11.7 | 9.9 | 8.6 | 7.6 |
| WX-205 | 34.0 | 12.4 | 10.5 | 9.1 | 8.0 |
| WX-250 | 44.0 | 16.1 | 13.6 | 11.8 | 10.4 |
| WX-251 | 62.0 | 22.7 | 19.2 | 16.6 | 14.6 |
| WX-255 | 81.0 | 29.6 | 25.0 | 21.7 | 19.1 |
| WX-302 | 86.0 | 31.4 | 26.6 | 23.0 | 20.3 |
| WX-350 | 119.0 | 43.5 | 36.8 | 31.9 | 28.1 |
Factory precharge 38 psig; maximum working pressure 150 psig. Where the documents differ: The older MC7025 (04/14) card lists the WX-103 at 7.6 gallons (2.4 / 2.0 / 1.8 gal drawdown) and a WX-252 (86 gal, 100 psig max). The 03/24 card lists the WX-103 at 6.7 gallons and drops the WX-252. We use the 03/24 card. Quick-sizing table, 30 GPM / 30/50 / 2-minute column: Amtrol prints a single WX-350 (36.8 gal drawdown), smaller than its own 25 GPM pick of two WX-255s. Shown as printed. WX-101 at 40/60: 2.0 gal × the Boyle factor 0.268 = 0.54 gal, but Amtrol prints 0.6 gal (the same as at 30/50). WX-200/201 at 40/60: 14.0 × 0.268 = 3.75 gal, printed 3.8. Every other printed drawdown matches volume × factor to the nearest 0.1 gal.
Amtrol's quick-sizing picks
| GPM | 30/50, 1 min | 40/60, 1 min | 50/70, 1 min | 30/50, 2 min | 40/60, 2 min | 50/70, 2 min |
|---|---|---|---|---|---|---|
| 5 | WX-202 | WX-202 | WX-202XL | WX-205 | WX-250 | WX-250 |
| 7 | WX-202XL | WX-203 | WX-203 | WX-251 | WX-251 | WX-251 |
| 10 | WX-203 | WX-250 | WX-250 | WX-255 | WX-255 | WX-302 |
| 12 | WX-250 | WX-251 | WX-251 | WX-255 | WX-350 | WX-350 |
| 15 | WX-251 | WX-251 | WX-255 | WX-350 | WX-350 | WX-255 (2) |
| 20 | WX-255 | WX-255 | WX-302 | WX-255 (2) | WX-255 (2) | WX-302 (2) |
| 25 | WX-255 | WX-350 | WX-350 | WX-255 (2) | WX-350 (2) | WX-350 (2) |
| 30 | WX-350 | WX-350 | WX-255 (2) | WX-350 | WX-350 (2) | WX-302 (3) |
| 35 | WX-350 | WX-255 (2) | WX-255 (2) | WX-350 (2) | WX-350 (3) | WX-350 (3) |
| 40 | WX-255 (2) | WX-255 (2) | WX-302 (2) | WX-350 (3) | WX-350 (3) | WX-350 (3) |
"(2)" = two tanks. Amtrol labels the GPM column "approximate".
Goulds HydroPro / CentriPro V-series
Printed values from Goulds TTECHWP R6 p.7. Columns are precharge/cut-out (28/50 = a 30/50 switch). Goulds counts the extra 2 psi, so these run about 15% higher.
| Model | Total volume (gal) | 18/40 | 28/50 | 38/60 |
|---|---|---|---|---|
| V6P | 2.0 | 0.8 | 0.7 | 0.6 |
| V15P | 4.5 | 1.8 | 1.5 | 1.3 |
| V25P | 8.2 | 3.3 | 2.8 | 2.4 |
| V45 / V45B / V45P | 13.9 | 5.6 | 4.7 | 4.1 |
| V60 / V60B | 19.9 | 8.0 | 6.8 | 5.8 |
| V80 / V80EX | 25.9 | 10.4 | 8.8 | 7.6 |
| V100 / V100S | 31.8 | 12.8 | 10.8 | 9.4 |
| V140 / V140B | 45.2 | 18.2 | 15.4 | 13.3 |
| V200 / V200B | 65.1 | 26.2 | 22.1 | 19.2 |
| V250 | 83.5 | 33.6 | 28.4 | 25.6 * |
| V260 | 84.9 | 34.1 | 28.9 | 25.0 |
| V350 | 115.9 | 46.6 | 39.4 | 34.1 |
* V250 at 38/60: Goulds prints 25.6 gal, but 83.5 gal × its own 0.295 factor is 24.6 gal (the 18/40 and 28/50 values for the V250 do match). The calculator uses 24.6 for matching and shows the printed 25.6 in the chart with a flag.
When this calculator does not apply
- Constant-pressure (variable-speed) drives need a smaller tank and their own precharge: Generic VFD tank sizing: tank volume 10 to 20% of the pump's output rating at 60 Hz (100 GPM → 10–20 gal tank). (Franklin AIM 2026, p.22, VFD Pressure Tank Sizing). Follow the drive's manual.
- Water used while the pump refills stretches the run; the result is a minimum.
- Low-yield wells, cycle-stop valves, galvanized tanks without a bladder follow other rules.
Related
- Changing the switch or precharge: pressure switch settings: 30/50 vs 40/60.
- Not sure of your pump's GPM: well pump size calculator.
- Pump clicking on and off every few seconds: well pump troubleshooting.
Frequently asked questions
What size pressure tank do I need for a 10 gpm pump?
For one minute of run time, the tank must deliver 10 gallons between starts. With the precharge 2 psi below cut-in that is about 34 gallons of total volume at 30/50 and 39 gallons at 40/60.
Is a bigger pressure tank better?
A bigger tank means fewer pump starts. Franklin Electric limits single-phase motors to 300 starts a day up to 3/4 HP and 100 a day from 1 HP; excess cycling shortens switch, relay and capacitor life. The trade-off is cost and space.
What should the pressure tank precharge be?
2 psi below the switch cut-in, measured at the air valve with the pump off and the tank drained: 28 psi for 30/50, 38 psi for 40/60. Constant-pressure drives use their own tables.
Sources
- Amtrol, Well-X-Trol specification and ESP quick-sizing card MC7025 (03/24), pp.1–2 · checked October 5, 2026
- Amtrol, Sizing the Well-X-Trol 9017-102 (03/19), pp.2–4 · checked October 5, 2026
- Amtrol, Well-X-Trol Residential Installation & Operation Instructions 9016-868 MC10185 (02/19), p.3 · checked October 5, 2026
- Goulds Water Technology (Xylem), Water Products Technical Data and Pump Fundamentals TTECHWP R6 (December 2022), p.7 (Table 1 tank models, Table 2 pressure factors, 'extra 2 psi' factors) · checked October 5, 2026
- Franklin Electric, AIM Manual 60 Hz Submersible Systems, 2026 edition (M1311 06-26), p.4 (Frequency of Starts: starts table, minimum run time), p.22 (VFD Pressure Tank Sizing) · checked October 5, 2026
- Franklin Electric, SubDrive/MonoDrive Constant Pressure Controller Installation Manual M1560 (225417101 Rev 7, 05-13), p.29 (Pressure Tank; Table 5 'Pressure Tank Pre-charge') · checked October 5, 2026
- Franklin Electric, SubDrive Connect Owner's Manual 226203103 (©2024), p.15 (Minimum Pressure Tank and Supply Pipe Sizing) · checked October 5, 2026
- Square D Data Bulletin 9013DB0701 (12/2007), p.2 · checked October 5, 2026
Method: Drawdown factors are Boyle's-law arithmetic with absolute pressure (gauge + 14.7 psi), checked against Amtrol's printed drawdowns and Goulds' pressure-factor table. Tank volumes and drawdowns are copied from Amtrol MC7025 (03/24) and Goulds TTECHWP R6. Run-time rules are from Franklin Electric and Amtrol. We have not tested any tanks.