Pressure Tank Size Calculator

Calculate the required pressure tank (accumulator tank) size for a water pump system based on your flow rate, cut-in/cut-out pressures, and desired pump cycle time.

Maximum flow demand in gallons per minute
Pressure at which the pump turns ON (typically 30 or 40 PSI)
Pressure at which the pump turns OFF (typically 50 or 60 PSI)
Minimum time between pump starts (1–2 min recommended to protect motor)
Air pre-charge pressure (typically 2 PSI below cut-in pressure)

Formulas Used

1. Drawdown Volume (Vd):

Vd = (Q × T) / 4

  • Q = Peak flow rate (GPM)
  • T = Minimum pump cycle time (minutes)
  • Dividing by 4 accounts for the worst-case scenario where the pump runs 50% of the cycle time

2. Total Tank Volume (Vt) — Boyle's Law with Pre-Charge:

Vt = Vd × (P1a × P2a) / (Ppa × (P2a − P1a))

  • P1a = Cut-in pressure + 14.7 (PSIA)
  • P2a = Cut-out pressure + 14.7 (PSIA)
  • Ppa = Pre-charge pressure + 14.7 (PSIA)

This formula applies Boyle's Law (P1V1 = P2V2) to the air bladder/diaphragm inside the pressure tank to determine the total tank volume needed to deliver the required drawdown volume.

Assumptions & References

  • The pressure tank uses a bladder or diaphragm design (most modern tanks); the air charge is isolated from the water.
  • Pre-charge pressure should be set to 2 PSI below the cut-in pressure (e.g., 28 PSI for a 30 PSI cut-in) per manufacturer recommendations.
  • Atmospheric pressure is assumed to be 14.7 PSI (sea level); adjust for high-altitude installations.
  • The worst-case pump duty cycle is 50% (pump on for half the cycle time), which is the standard design assumption per Hydraulic Institute standards.
  • Minimum pump cycle time of 1–2 minutes is recommended to prevent motor overheating from frequent starts.
  • Standard tank sizes follow common North American pressure tank product lines (Well-X-Trol, Amtrol, Flexcon, etc.).
  • Formula source: Amtrol Inc. Engineering Data, Hydraulic Institute Standards, and AWWA M45.
  • This calculator is for residential and light commercial water well systems. Industrial systems may require additional safety factors.

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