WATER HEATER SIZE
WATER HEATER SIZE
Per DOE FHR + GPM tables
RESULT
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Tank sizing per DOE Energy Saver FHR guidelines. Tankless GPM assumes 70°F temperature rise (typical for cold-climate winter inlet). Adjust GPM up 20-30% for inlet temps below 50°F.
About this calculator
This water heater sizing calculator returns the recommended residential water heater capacity for either a tank-style heater (rated in gallons) or a tankless heater (rated in GPM at a 70°F temperature rise). The math follows DOE Energy Saver guidelines based on household size, the strongest predictor of peak-hour hot water demand. For tank heaters, the recommendation accounts for First Hour Rating (FHR) — the gallons of hot water available in the busiest hour of the day. For tankless, the GPM target assumes one shower plus a bathroom sink running simultaneously at the household's typical peak.