Pressure tank sizing calculator
Size a well-pump pressure tank by the drawdown method: pump flow × minimum run time, divided by the drawdown factor for your pressure switch and precharge. Every factor is computed from Boyle's law, not read from a rounded table.
Use the pump's flow at the pressure-switch range, not its open-discharge rating — the pump curve or the installer's flow test gives it. The precharge follows the switch setting (cut-in − 2 psi) until you type your own.
How the size is worked out
drawdown = pump gpm × minimum run time (min)DF = (Ppre + 14.7) ÷ (Pin + 14.7) − (Ppre + 14.7) ÷ (Pout + 14.7)V = drawdown ÷ DFWith the precharge equal to cut-in, the factor simplifies to Amtrol's published form, AF = 1 − (Pin + 14.7) ÷ (Pout + 14.7). Pressures are gauge (psig); 14.7 converts them to absolute.
Pentair's worked example. An 8 gpm pump on 30/50 for 1 minute: 8 ÷ 0.30 = 26.7 gal minimum. The exact factor is 0.309, giving 25.9 gal; Pentair rounds the factor down, which errs toward the bigger tank.
| Switch | Published (precharge = cut-in) | Amtrol precharge | Factor at that precharge | Tank for 10 gpm × 1 min |
|---|---|---|---|---|
| 20/40 psi | 0.366 | 18 psi | 0.345 | 29.0 gal |
| 30/50 psi | 0.309 | 28 psi | 0.295 | 33.9 gal |
| 40/60 psi | 0.268 | 38 psi | 0.258 | 38.8 gal |
| 50/70 psi | 0.236 | 48 psi | 0.229 | 43.7 gal |
| 60/80 psi | 0.211 | 58 psi | 0.206 | 48.7 gal |
| Model | Total volume | 30/50 | 40/60 | 50/70 |
|---|---|---|---|---|
| WX-101 | 2 gal | 0.6 | 0.5 | 0.5 |
| WX-102 | 4.4 gal | 1.4 | 1.2 | 1.0 |
| WX-103 | 6.7 gal | 2.1 | 1.8 | 1.6 |
| WX-104 | 10.3 gal | 3.2 | 2.8 | 2.4 |
| WX-200 / 201 | 14 gal | 4.3 | 3.7 | 3.3 |
| WX-202 | 20 gal | 6.2 | 5.4 | 4.7 |
| WX-202XL | 26 gal | 8.0 | 7.0 | 6.1 |
| WX-203 | 32 gal | 9.9 | 8.6 | 7.6 |
| WX-205 | 34 gal | 10.5 | 9.1 | 8.0 |
| WX-250 | 44 gal | 13.6 | 11.8 | 10.4 |
| WX-251 | 62 gal | 19.2 | 16.6 | 14.6 |
| WX-255 | 81 gal | 25.0 | 21.7 | 19.1 |
| WX-302 | 86 gal | 26.6 | 23.0 | 20.3 |
| WX-350 | 119 gal | 36.8 | 31.9 | 28.1 |
The drawdown columns reproduce Amtrol's MC7025 card to one decimal place. Pentair's composite WellMate tanks follow the same Boyle's-law factors; steel tanks with a maximum acceptance factor must also stay inside it.
FAQ
TVC-403/FAQWhat size pressure tank do I need?
Multiply the pump's flow in gallons per minute by the minimum run time (Amtrol uses 1 minute for pumps up to ¾ hp and 2 minutes above), then divide by the drawdown factor for your pressure switch. A 10 gpm pump on a 40/60 switch with a 38 psi precharge needs 10 ÷ 0.258 ≈ 39 gallons of total tank volume.
What is drawdown?
The water the tank delivers between the pump shutting off at cut-out pressure and starting again at cut-in. It is only a fraction of the tank's total volume — about a quarter on a 40/60 switch — because the air cushion has to stay compressed.
Why does the pump need a minimum run time?
Every start heats the motor and wears the pressure switch and starting components. A tank big enough for a minute or more of pumping between starts keeps the number of starts per hour down. Check your pump maker's minimum — some ask for more than Amtrol's 1 and 2 minutes.
What should the pressure tank precharge be?
Amtrol's instructions: 2 psi below the pressure-switch cut-in, with the tank empty of water — 38 psi for a 40/60 switch, 28 psi for 30/50. Pentair ships its Pro-Source steel tanks at 40 psi for 40/60 systems; follow the instructions for the tank you have.
Why is the drawdown factor slightly smaller with the 2 psi lower precharge?
A lower precharge leaves a little water in the tank when the pump cuts in, and that water is never delivered. Manufacturers' published drawdown tables assume precharge equals cut-in; the calculator works from your actual precharge, so its figure is a few percent smaller and the recommended tank a little larger.
Sources and standards
- Well-X-Trol Commercial Sizing (9017-102, 03/19)AMTROL Inc. (manufacturer)https://www.amtrol.com/wp-content/uploads/2019/06/9017-102-03_19-WXT-Sizing.pdfAF = 1 − (P2 + 14.7) ÷ (P3 + 14.7); TV = ESP volume ÷ AF.
- Well-X-Trol Sizing Card (MC7025, 03/26)AMTROL Inc. (manufacturer)https://www.amtrol.com/wp-content/uploads/2026/03/MC7025-03_26-WXT-card.pdfModel total volumes and drawdowns at 30/50, 40/60 and 50/70; 1-minute run time up to ¾ hp, 2 minutes above.
- Well-X-Trol Installation & Operation Manual (9015-868 / MC10185, 07/25)AMTROL Inc. (manufacturer)https://www.amtrol.com/wp-content/uploads/2025/08/9015-868-MC10185-07_25-WXT-IO-manual.pdfSet the precharge 2 psig below the pressure-switch cut-in (38 psig for a 40/60 switch).
- WellMate Composite Pressure Tank Catalog (WM900, 09-01-23), drawdown factor tablePentair (manufacturer)https://pentair.com/content/dam/extranet/web/nam/wellmate/catalogs/wm900-composite-pressure-tank-cat.pdfDrawdown factors based on Boyle's law (e.g. .309 at 30/50, .268 at 40/60); worked example 8 gpm × 1 min ÷ .30 = 26.7 gal.
- Pro-Source Steel Pressure Tanks Data Sheet (M9134WS, 09-15-2021)Pentair (manufacturer)https://flowandfiltrationsolutions.pentair.com/content/dam/extranet/web/nam/pro-source/data-sheets/m9134ws-steel-tanks-ds.pdf“Size tank for one gallon of drawdown for each gallon per minute at pump capacity.”
Every linked source above was retrieved and checked on 2026-11-01.