Tool

Well Pressure Tank Size Calculator

Enter the pump's flow and your switch setting. You get the minimum total tank volume, the drawdown behind it, and — if you enter your tank — how long your pump runs per cycle.

By Well Pump Handbook EditorialPublished How we research

Pressure tank size calculator

This tool is for educational guidance and does not replace an onsite diagnosis. Method: Water Well Journal (minimum run time and drawdown) and The Driller (Boyle's law with absolute pressures). Assumes constant pump flow and ignores temperature — round up, and check the tank maker's drawdown chart.

The method

  1. Drawdown needed = pump flow (GPM) × minimum run time. Water Well Journal uses 1 minute for motors under 2 HP and 2 minutes at 2 HP and above.[1] Franklin Electric also asks for at least 1 minute of running per start.[2]
  2. Drawdown factor = P1 × (1/P2 − 1/P3), in absolute pressure: P1 = precharge, P2 = cut-in, P3 = cut-out, each plus atmospheric pressure (14.7 psi at sea level). This is Boyle's law as published in The Driller.[3]
  3. Minimum total volume = drawdown needed ÷ drawdown factor.

The result also shows Water Well Journal's simpler factor, 1 − (cut-in + 14.7) ÷ (cut-out + 14.7), which assumes the precharge equals the cut-in. With the usual precharge 2 psi below cut-in, our number comes out slightly larger.

Elevation: if you enter one, atmospheric pressure is estimated with the U.S. Standard Atmosphere formula. The Driller's example shows altitude increases drawdown slightly (about 1 gallon on an 85-gallon tank at 6,000 ft).[3]

Other well calculators: see all tools.

Check the answer

Water Well Journal's published example — 18 GPM, 2 HP, 45/65 psi — needs 36 gallons of drawdown and a 144-gallon tank using their formula.[1] Enter 18 GPM, "2 HP or more", a custom 45/65 setting and a 45 psi precharge to reproduce it.

Assumptions and limits

  • Pump flow is treated as constant through the cycle; real pumps deliver a little less near cut-out.
  • Temperature and gauge error are ignored.
  • Tank models have a maximum share of their volume that can be water; Amtrol says to check it.[4] Residential settings are normally inside it.
  • Constant-pressure systems use different tank rules — see constant-pressure systems.

This tool is for educational guidance and does not replace an onsite diagnosis.

Sources

  1. Water Well Journal. Tank Selection by the Numbers — David L. Kill, PE (April 19, 2022). Accessed Sept. 28, 2026.
  2. Franklin Electric. Submersible Motors: Application, Installation & Maintenance (AIM) Manual, 60 Hz. Accessed Sept. 28, 2026.
  3. The Driller. How to Calculate and Control Pressure Tank Drawdown — Bob Pelikan (Feb. 1, 2006). Accessed Sept. 28, 2026.
  4. Amtrol. Sizing the Well-X-Trol (commercial sizing handbook). Accessed Sept. 28, 2026.

We cite manufacturer documentation and public agencies first. Read how we choose and check sources.

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