Well Pump Pressure Tank Size Calculator

Calculate the minimum pressure tank size needed for your well pump system to prevent short cycling and extend pump life.

Pump Flow Rate (GPM) Gallons per minute rating of your well pump

Cut-In Pressure (PSI) Pressure at which the pump turns ON (commonly 30 or 40 PSI)

Cut-Out Pressure (PSI) Pressure at which the pump turns OFF (commonly 50 or 60 PSI)

Maximum Pump Cycles per Hour Recommended: 6 cycles/hr for most pumps (check manufacturer specs)

Tank Pre-Charge Pressure (PSI) Air pre-charge pressure (typically 2 PSI below cut-in pressure)

1. Drawdown Volume (Vd):

Vd = Q ÷ (4 × N)

2. Acceptance Ratio (Ar) — using Boyle's Law:

Ar = (P_pre / P_cut-in) × (1 − P_cut-in / P_cut-out)

All pressures converted to absolute (PSIA) by adding 14.7 PSI atmospheric pressure.

3. Minimum Tank Volume (Vt):

Vt = Vd ÷ Ar

4. Recommended Tank Volume:

V_recommended = Vt × 1.25 (25% safety margin)

Based on the Cycle Stop Valves / Pressure Tank Sizing standard method using Boyle's Law for bladder/diaphragm tanks.
The drawdown formula (Q / 4N) is derived from the standard pump cycling equation where maximum heat dissipation occurs at 50% duty cycle.
Atmospheric pressure assumed at 14.7 PSI (sea level). Adjust for high-altitude installations.
Pre-charge pressure should be set 2 PSI below cut-in pressure with the tank empty of water.
Common residential cut-in/cut-out settings: 30/50 PSI or 40/60 PSI.
Recommended maximum cycles: 6 per hour for most 1/2–1 HP submersible pumps; consult manufacturer for exact limits.
Results apply to bladder or diaphragm pressure tanks. Galvanized (air-over-water) tanks require different sizing.
Reference: Goulds Water Technology Pump Selection Guide; Amtrol Well-X-Trol Sizing Guide; AWWA Manual M21.
Always select the next larger standard commercial tank size above the calculated minimum.

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