Expansion Tank Calculator

Size a thermal expansion tank for a closed (no-backflow) plumbing or hydronic system.

airexpansion tank
TANK SIZE
— gal
minimum

Usage Tip

Pre-charge the tank air pressure to match your supply pressure before installing.

THE MATH
V = sysVol × 0.052 ÷ (1 − P ÷ Pmax)
Where: P = supply pressure  •  P max = relief valve setting
• Required on closed systems with a check valve or PRV
• Round up to the next tank size
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.