Pressure Tank Size Calculator
Estimate required water-system drawdown, nominal storage capacity, selected tank quantity, purchased volume and cost.
Default project preview
55 gal
Required usable water
16.5 gal
Selected tanks
2
Purchased nominal volume
88 gal
Purchased usable water
26.4 gal
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Pump flow rate
Enter this project assumption in gal/min. The current imperial default is 10 gal/min.
Minimum pump runtime
Enter this project assumption in minutes. The current imperial default is 1.5 minutes.
Demand reserve
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Usable water fraction
Enter this project assumption in %. The current imperial default is 30 %. Minimum: 1. Maximum: 100.
Selected tank nominal volume
Enter this project assumption in gal. The current imperial default is 44 gal.
Price per selected tank
Enter this project assumption in a unitless value. The current imperial default is 420. Minimum: 0.
How to read the outputs
Required usable water
flow × runtime × reserve. Default preview: 16.5 gal.
Selected tanks
44 gal each. Default preview: 2.
Purchased nominal volume
full selected tanks. Default preview: 88 gal.
Purchased usable water
30% entered fraction. Default preview: 26.4 gal.
Usable-water margin
above requirement. Default preview: 9.9 gal.
Tank purchase cost
selected tanks only. Default preview: $840.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Required usable water = pump flow × minimum runtime × reserve factor. Required nominal capacity = usable water ÷ entered usable fraction.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to estimate pressure-tank nominal volume from pump flow, desired minimum runtime and drawdown fraction. It does not select pressure settings, tank rating or pump controls.
Calculation steps
- 1Determine pump flow at the actual system head.
- 2Choose the required minimum runtime or cycle volume.
- 3Find drawdown fraction for the selected cut-in and cut-out settings.
- 4Divide required drawdown by the fraction and round to a standard tank.
Worked numerical example
A 10 gpm pump with a desired one-minute minimum runtime needs 10 gallons of drawdown. If the selected pressure range provides 30% drawdown, nominal tank volume is about 33.3 gallons before standard-size selection.
Purchasing and rounding
Tank label volume is not usable drawdown. Compare manufacturer drawdown tables for the exact precharge and pressure-switch settings.
Interpret the result
A larger tank generally lengthens pump cycles but does not increase well yield. Variable-speed systems use different control and storage assumptions.
Page-specific assumptions
- Pump flow is known.
- Pressure settings are stable.
- Drawdown fraction matches the tank data.
- No unusual demand pattern or variable-speed control.
Common mistakes
- Equating tank gallons with drawdown gallons.
- Using pump nameplate flow at zero head.
- Setting precharge incorrectly.
- Using tank size to compensate for a low-yield well.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Pressure Tank Size Calculator questions
Why is nominal capacity larger than usable water?+
Only part of the rated tank volume is delivered during a normal operating cycle.
Can multiple smaller tanks be combined?+
Often yes, when the piping and controls are designed for parallel tanks.
Where should the usable fraction come from?+
Use the selected manufacturer’s published drawdown data for the operating settings.