Water Heater Size Calculator
Calculate the right water heater size for your home. Find recommended tank size in gallons or tankless flow rate in GPM based on occupants, bathrooms, and climate.
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Recommended Tank Size
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Recommended Tankless Flow Rate —
Target First Hour Rating (FHR) —
Sizing Recommendation —
Extended More scenarios, charts & detailed breakdown ▾
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gal
Recommended Tank Size
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Required First Hour Rating —
Required Recovery Rate —
Tank Selection Note —
Professional Full parameters & maximum detail ▾
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°F
$/therm or $/kWh
Sizing
Recommended Tank Size —
Target First Hour Rating —
Tankless Flow Rate Required —
Peak Demand (simultaneous fixtures) —
Operating Cost
Est. Annual Operating Cost —
Est. Installation Cost —
Recommendation
System Type Recommendation —
How to Use This Calculator
- Enter the number of bathrooms and occupants in your home.
- Select your climate (affects tankless sizing for incoming water temperature).
- Results show recommended tank size in gallons and tankless flow rate in GPM.
- Use the Tank tab for detailed FHR calculations. Use the Tankless tab to size by simultaneous fixtures.
Formula
Tank Size: Base gallons = Occupants x 12 gal/person x Climate Factor
Tankless GPM: Sum of simultaneous fixture flow rates (shower: 2 GPM, dishwasher: 1.5 GPM, washing machine: 2 GPM)
Example
Example: 3 occupants, 2 bathrooms, moderate climate. Tank: 3 x 12 x 1.1 = ~40 gal (round up to 50-gal standard size). Tankless: 2 showers + 1 dishwasher = 2 + 2 + 1.5 = 5.5 GPM.
Frequently Asked Questions
- A family of 4 typically needs a 50-60 gallon tank water heater. For tankless, you need roughly 7-9 GPM flow rate to handle simultaneous showers and appliances. Cold climates require more capacity due to lower incoming water temperature.
- First Hour Rating measures how many gallons of hot water a tank heater can deliver in the first hour of use, starting with a full tank. It is a more useful sizing metric than tank size alone. Match FHR to your peak demand hour.
- Add up the flow rates of all fixtures that might run simultaneously. A shower uses 2-2.5 GPM, a dishwasher 1.5 GPM, a washing machine 2 GPM. Size the unit for your peak simultaneous demand.
- Heat pump water heaters are 2-3x more efficient than electric resistance heaters, typically saving $300-$500/year in energy costs. They have higher upfront costs ($1,200-$2,000) but often pay back in 3-5 years.
- Cold climates require more capacity because incoming water temperature can be 40-50°F instead of 60-70°F in warm climates. The heater must work harder to reach the desired 120-140°F setpoint, reducing effective first-hour capacity.