Water Heater Calculator — Tank Size & Flow Rate by Household
Tank Size
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First Hour Rating
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Peak Flow (GPM)
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Temp Rise
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Water Heater Sizing
1-2 people: 30-40 gal. 3-4: 40-50. 5+: 50-80. FHR should match peak demand.
Tankless: size by flow rate and temp rise. Cold climates need larger units.
Shower: 2 GPM, bath: 4, dishwasher: 1.5. Peak simultaneous usage estimated.
Frequently Asked Questions
What size for a family of 4?
40-50 gallon tank with First Hour Rating of 60-80 gallons, or tankless with 5-7 GPM.
What is First Hour Rating?
Gallons of hot water a tank heater delivers in one hour starting full. Combines capacity with recovery rate.
Tank vs tankless?
Tank costs less upfront. Tankless costs more but lasts longer, uses less energy, and provides unlimited hot water.
How does inlet temperature affect sizing?
Colder inlet water needs greater temperature rise. Northern groundwater (40-50F) vs southern (65-75F).