Free Engineering Tool · NFPA 22
Fire Sprinkler Pressure Tank Calculator
Size a pressure tank for a sprinkler system: the air pressure needed so the last of the water still leaves at the required pressure, or the largest air fraction that keeps the air below a pressure limit, together with the water volume and the total tank volume.
- Based on NFPA 22
- Free, runs in your browser; your inputs stay on your device
- Screening tool, not reviewed or sealed by a licensed engineer
- Last updated October 2026
Screening tool, not engineering advice. This calculator estimates results with simplified published methods. It has not been reviewed or sealed by a licensed engineer for your project, and DGA does not warrant that its results are accurate, complete or suitable for any use. Check every result independently and have the licensed professional responsible for your project review it before you rely on it. Use is subject to the Terms of Use.
System Demand
Reference point: the highest sprinkler at its minimum pressure (15 psi in the traditional pipe schedule rule), or the point where a hydraulic calculation gives the demand pressure. Use a negative height if the point is below the tank bottom.
Tank
NFPA 22 rounds atmospheric pressure to 15 psi (14.7 psi at sea level; lower at altitude, which raises the air pressure needed).
Result
Tank and Pressure While Draining
About This Tool
A pressure tank holds water with compressed air above it. As water leaves, the air expands and its pressure falls. NFPA 22 requires enough air at a high enough pressure to push all of the water out of the tank while still giving the required pressure at the top of the system. When the tank is empty the air fills the whole tank, so Boyle’s law at constant temperature, using absolute pressures, gives (Pi + Pa) A = Pf + Pa, or Pi = (Pf + Pa) / A − Pa. Pi is the gauge air pressure to maintain, A is the fraction of the tank that is air at the start, Pf is the gauge pressure needed at the tank when it is empty and Pa is atmospheric pressure, which NFPA 22 rounds to 15 psi.
Pf is the pressure required at a reference point plus 0.433 psi for each foot that point sits above the tank bottom. For a pipe schedule system the reference is the highest sprinkler at 15 psi, which reproduces the traditional formulas P = 30/A − 15 and P = (30/A − 15) + 0.434H/A. With one-third air, the first gives the familiar 75 psi. For a hydraulically calculated system, use the demand pressure from the calculation at the point it refers to. Solving the same equation for A gives the largest air fraction that keeps the air pressure at or below a limit such as a 175 psi component rating. The water to store is the demand flow times the duration, and the tank volume is that water divided by (1 − A).
Worked example
A small system needs 150 gpm for 30 minutes and 15 psi at its highest sprinkler, 30 ft above the tank bottom. The pressure needed at the tank when it is empty is 15 + 0.433 × 30 = 28.0 psi. With the tank one-third air, the air pressure must be (28.0 + 15) / (1/3) − 15 = 114 psi, below a 175 psi rating. The water is 150 × 30 = 4,500 gal, so the tank must hold 4,500 / (2/3) = 6,750 gal. Holding the air at 175 psi instead would allow up to 22.6would allow up to 22 percent air and a 5,780 gal tanknbsp;percent air and a tank of about 5,820would allow up to 22 percent air and a 5,780 gal tanknbsp;gal.
Assumptions and limits
The method treats the air as an ideal gas expanding at constant temperature and assumes the full demand flows for the whole duration. Friction loss between the tank and the reference point must be included in the required pressure, as a hydraulic calculation does. Pressure tanks are limited by NFPA 13 and the authority having jurisdiction to specific uses and durations, and the tank, air supply, gauges and relief valve must meet NFPA 22. At altitude, enter the local atmospheric pressure; a lower value raises the air pressure needed.
When to bring in an engineer
A pressure tank is one of several ways to supply a system where the public water supply falls short. DGA compares tanks, pumps and other options as part of fire protection design and prepares the fire pump designs that are often the alternative.
References
- NFPA 22, Standard for Water Tanks for Private Fire Protection, National Fire Protection Association, 2023 edition: pressure tanks, including the air quantity and pressure needed to expel all of the water at the required residual pressure.
- NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, 2025 edition: pressure tanks as a water supply, duration and the elevation pressure of 0.433 psi/ft.
- Y.A. Çengel and M.A. Boles, Thermodynamics: An Engineering Approach, McGraw-Hill: ideal gas behavior and Boyle’s law for an isothermal process.
Frequently Asked Questions
Why is a pressure tank usually kept two-thirds full of water?
With one-third of the tank as air at 75 psi, the air expands to fill the whole tank and still holds 15 psi at the tank as the last water leaves (30 / (1/3) – 15 = 75). That covers a tank at the level of the highest sprinkler. Tanks below the sprinklers, or systems with higher demand pressures, need more air or more pressure.
How do I find the air pressure for a hydraulically calculated system?
Take the demand pressure from the hydraulic calculation, add 0.433 psi per foot for the height of that point above the tank bottom, add atmospheric pressure (15 psi), divide by the air fraction and subtract 15 psi. For a 60 psi demand at the tank and half the tank as air, (60 + 15) / 0.5 – 15 = 135 psi.
How big does the tank need to be?
The water volume is the demand flow times the duration, and it can occupy only the part of the tank that is not air. With 4,500 gal of water and one-third air, the tank must hold 4,500 / (2/3) = 6,750 gal.
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Engineering aid only, provided as is and without warranty. Results depend on the inputs and on the simplifying assumptions of the method summarized on each tool page, and DGA has not verified them for your application. They do not replace code-required design, testing or review by the licensed engineer responsible for the project. Use of the tools is subject to the Terms of Use.
Related Reading
Articles from DGA Insights on the code and the method behind this calculator.