Water Heater sizing
Water Heater Sizing Guide & Calculator
Find the right water heater size for your household. Our DOE FHR-based calculator calculates the exact gallon capacity or tankless flow rate you need, and our guides cover tank vs tankless, household size, and usage patterns.
Last reviewed Sources: AHAM & DOE Standards Reviewed by the WhatSize editorial team
Water Heater sizing guides (8)
Standards reference
All water heater sizing recommendations on this site are based on DOE First Hour Rating (FHR) — 10 CFR 430 • ENERGY STAR Water Heater Criteria • AHRI Standards and related industry standards.
Last reviewed: • Editorial review cadence
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Most-asked questions about water heater sizing
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.