Pressure Tank Sizing Calculator

What this tool does

Convert an entered minimum pump run time into required drawdown and idealized nominal tank volume. Optionally check a manufacturer-rated drawdown against that requirement without confusing usable water with the tank's nominal capacity.

When to use it

Use for preliminary conventional on/off pump systems, not VFD/constant-pressure controls or thermal-expansion vessels. Obtain the pump's delivery at the relevant pressure and the pump/motor manufacturer's run-time requirement; the example's 1 minute is not a universal recommendation.

Inputs and supported units

Each labelled field explains the quantity it expects. Unit-system changes convert existing numeric entries once and keep calculations on a single SI internal basis. Supported shared units include L/min and GPM, metres and feet, kPa and psi, millimetres and inches, kW and hp, and °C and °F as relevant.

Calculation or decision method

Gauge pressures are converted to absolute by adding atmospheric pressure. The tool requires precharge < cut-in < cut-out. The original ideal-gas sizing model is unchanged.

Compare rated drawdown, not nominal capacity

1. Establish the requirement. Use pump delivery at the relevant duty/pressure, not a household fixture count, motor horsepower alone or unrestricted faucet flow. A changing pump curve needs a manufacturer-reviewed sizing flow or conservative operating cases; this tool does not model the full curve.
2. Read the correct rating. Find usable drawdown between the actual cut-out and cut-in pressures in the current tank data. Check precharge, tank configuration and other stated rating conditions. A catalogue's nominal litres or gallons are not drawdown.
3. Confirm the basis before comparing. Choose the manufacturer-rated comparison, enter its drawdown and confirm the rating conditions match. If conditions differ, leave the comparison unconfirmed and obtain the correct rating; do not interpolate a product table or change precharge to force a favorable result.
4. Review the margin. Rated drawdown minus required drawdown gives a surplus or shortfall. Equality meets only the entered arithmetic target. Pressure/temperature suitability, relief protection, motor start limits and final selection still require the applicable manufacturer instructions and qualified review.

The refill estimate assumes constant inflow and no simultaneous outlet demand. It is not a guaranteed minimum actual run time, a starts-per-hour analysis, or a diagnosis of a waterlogged tank. Use the existing Short-Cycling Analyzer for a separate demand/cycle screen.

How to interpret the result

Read the primary result with the supporting breakdown and warning. A plausible number is not evidence that every input, operating case or design constraint has been included.

Assumptions

Ideal-gas behavior, fixed atmospheric pressure and a conventional captive-air tank without thermal or bladder-volume corrections. Optional drawdown is entered from current manufacturer data at matching conditions; the user's confirmation is not independent verification. Pump inflow is treated as constant only for the no-demand refill estimate.

Limitations

No automatic model selection, product database, VFD sizing, thermal-expansion sizing or pressure-vessel approval. The tool does not verify the supplied rating, prescribe precharge or run time, model varying pump delivery, account for outlet demand during refill, or replace motor start limits and current manufacturer instructions.

Sources

• Pentair — Steel Pressure Tanks Owner’s Manual
• Pentair — Pro-Source capacity, drawdown and sizing data
• Amtrol — Well-X-Trol model specifications and pressure-specific drawdown
• Hydraulic Institute — Pump FAQs