Water Heater sizing
Water Heater Sizing Guide & Calculator
A family of 4 uses about 70 gallons of hot water in their peak hour (showers + dishwasher + laundry). A 40-gallon electric tank can only deliver 50 gallons in that hour — meaning the last shower runs cold. Match First Hour Rating to peak demand, not tank capacity to household size.
Did you know? Electric tanks deliver 10–15% less FHR than gas at the same gallon size. A 50-gal gas tank with a 40,000 BTU burner delivers FHR 80–90; the same gallon size on electric only delivers 65–75. The FHR is what matters, not the tank.
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Water Heater sizing FAQ
How do I choose the right water heater size?
Use the DOE First Hour Rating (FHR) standard. Estimate your peak-hour hot water use (showers, dishwasher, laundry, cooking) and match it to a tank's FHR. For 1-2 people: 30-40 gallon tank. For 3-4: 40-50 gallons. For 5+: 50-80 gallons. For tankless: size by simultaneous GPM demand. Use our water heater calculator.
What size water heater for a family of 4?
A family of 4 typically needs a 40-50 gallon tank or a 7-9 GPM tankless. With average use (2 showers + dishwasher + laundry), peak-hour demand is 60-80 gallons of hot water. A 50-gallon gas tank with a 40,000 BTU burner typically delivers FHR of 80-90 gallons — enough for 4 people. Electric tanks deliver 10-15% less FHR due to slower recovery.
Tank or tankless water heater?
Tank water heaters: $800-1,500, last 10-15 years, simple to install, no flow rate limit within tank capacity, higher standby energy loss. Tankless water heaters: $1,500-3,500 installed, last 20+ years, endless hot water, 22-34% more energy efficient, but require gas line upgrade (for gas models) or 200-amp electrical service (for electric). Whole-house gas tankless is the highest-efficiency option in most US homes.
How much does a water heater cost to run?
A standard 40-gallon electric water heater uses 4,500 watts when heating, costing about $0.60/hour of use. A gas water heater costs $0.30-0.40/hour equivalent. Annual operating cost: $200-400 for electric, $150-300 for gas, $50-150 for tankless gas. ENERGY STAR heat pump water heaters cut electric costs by 60-70%, paying back in 5-7 years for households using 200+ gallons/day.
How long does a water heater last?
Standard tank water heaters last 10-15 years; tankless models last 20-25 years. Hard water shortens lifespan by 30-50%. Annual maintenance (flushing sediment, testing pressure relief valve) extends life by 2-3 years. Replace proactively at 12 years for tanks (before leaks cause $5,000+ water damage) or when recovery slows significantly. Look for rust-colored water or popping sounds as failure signs.
What size water heater for a 2-person household?
A 2-person household typically needs a 30-40 gallon tank or a 5-6 GPM tankless. With low-to-moderate use (1-2 showers, occasional dishwasher), peak-hour demand is 30-50 gallons. A 30-gallon electric tank works for budget-conscious singles; a 40-gallon gas tank provides headroom for guests. Tankless is overkill for 1-2 person households unless endless hot water is a priority.
Standards used: DOE First Hour Rating (FHR) — 10 CFR 430 • ENERGY STAR Water Heater Criteria • AHRI Standards • Editorial policy • Methodology
Looking for tankless-only? See our tankless water heater hub for a tankless-focused view of GPM calculations, fuel-type comparisons, and cold-climate sizing.
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