Construction · private well water
Well pressure tank sizing calculator.
Enter your pump flow, run time and pressure switch setting to see the drawdown you need, the acceptance factor, and the total tank volume. Runs in your browser; nothing is sent anywhere.
Planning aid: the Water Systems Council and tank makers advise confirming tank size and pressure settings with the manufacturer or a licensed well contractor.
Pump and pressure switch
Results
Worked example
A 10 GPM pump, 1 minute minimum run time, 30/50 psig switch. Precharge = 30 − 2 = 28 psig. Acceptance factor = 1 − (28 + 14.7) / (50 + 14.7) = 0.340. Drawdown needed = 10 × 1 = 10 gal. Total tank volume = 10 / 0.340 = 29.4 gal, so a nominal tank of roughly 30 gal or more. A 34 gal tank would deliver 34 × 0.340 = 11.6 gal between cycles by this formula.
Formulas, sources, and limits
Formulas: drawdown needed (gal) = pump GPM × minimum run time (min). Acceptance factor = 1 − (precharge + 14.7) / (cut-out + 14.7), all pressures in psig, with precharge set 2 psig below cut-in. Total tank volume = drawdown / acceptance factor. Usable drawdown of a given tank = nominal volume × acceptance factor. Run time at no household demand = usable drawdown / pump GPM.
Sources (read 2026-09-30): the acceptance factor, total-tank-volume formula and the 2 psig precharge rule are from the Water Systems Council wellcare sheet Sizing a Pressure Tank. That sheet refers to its Table IV.1.2 and the Water Systems Handbook for minimum drawdown by pump capacity; those tables are not reproduced here. The 1 minute (pumps up to 3/4 hp) and 2 minute (pumps over 3/4 hp) run times, and the model drawdowns below, are from Amtrol's Well-X-Trol card MC7025 (03/26). Amtrol's installation manual also sets precharge 2 psig below cut-in.
Amtrol model match: when your switch is exactly 30/50, 40/60 or 50/70, the results list the smallest Well-X-Trol model on that card whose listed drawdown meets your requirement, or several of the largest. Amtrol's drawdowns are slightly lower than the simple formula gives (for example 6.2 gal listed for the 20 gal WX-202 at 30/50 versus 6.8 gal by formula) and its own quick sizing table rounds to approximate GPM, so it can pick a slightly different model. Other brands will differ; check their tables.
Limits: assumes typical household demand and a pump and well that can meet it. It does not cover supplemental drawdown for pumps that cannot meet peak demand, low-yield wells, commercial or farm systems, altitude, or temperature. Pressures are gauge psig.
Frequently asked questions
What size pressure tank do I need for my well?
Pump GPM times minimum run time gives drawdown; divide by the acceptance factor to get total tank volume. The results above do both.
What is the acceptance factor?
The share of a tank's total volume that becomes available water between pump cycles, per the Water Systems Council sheet. Higher pressure spreads between cut-in and cut-out raise it.
Why is usable drawdown less than the tank's nominal size?
The air charge occupies part of the tank. Only the acceptance-factor fraction of nominal volume is delivered between cycles.
Does this replace a well contractor?
No. Both sources say to consult the tank manufacturer or a licensed well contractor for final sizing and pressure settings.