Water Heater Size Calculator

Water heater sizing starts from peak hour demand, which is the most hot water your household draws in any single hour. Usually that is the morning, with everyone showering back to back.

Peak Hour Demand = Sum of all hot water activities during the busiest hour

Unit conversions:

• Imperial: Gallons
• Metric: Liters (1 gallon = 3.785 liters)

Hot water usage per activity:

Sizing guidelines by household:

This is the table the calculator starts from. Peak hour demand is reported alongside it as context, not as the tank size, and confusing the two is the mistake almost every online sizing guide makes. Where your peak demand outruns what that size can actually deliver in an hour on your fuel, the calculator steps up one size and says why. That mostly happens on electric, and on households of five or more.

Why peak demand is bigger than the tank A 4-person household on average use has a peak hour demand around 72 gallons, yet the table recommends a 50-gallon tank. That is not a contradiction. The heater is refilling and reheating throughout that hour, so it delivers far more hot water in 60 minutes than it physically holds. The number that captures this is the First Hour Rating, printed on the yellow EnergyGuide label, and it is the figure you should actually compare against your peak demand.

Typical First Hour Rating by tank size:

Look down that table and the practical consequence jumps out: a 50-gallon gas heater covers a 72-gallon peak comfortably, while a 50-gallon electric one does not come close. Electric households routinely need to go one size up for the same number of people, which is exactly why the calculator asks which fuel you have.

Tips:

• Tankless water heaters are sized by flow rate (GPM), not tank size, so this table does not apply to them
• Gas recovers roughly twice as fast as electric, and that gap is the whole reason for the two columns above
• Undersizing by one step is the most common regret. Oversizing costs a few dollars a year in standby loss, which is the cheaper mistake