Water Heater Size Calculator

Estimate the water heater tank size and first-hour rating (FHR) for your household.

TANK SIZE
— gal
recommended

Usage Tip

Match the first-hour rating to your busiest hour of hot-water use, not just the raw tank size.

THE MATH
tank = people × gal/person × peak
Where: gal/person depends on heater type  •  FHR = people × gal/person
• FHR (first-hour rating) matters most when buying
• Round up to a standard 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.