Water Heater Size Calculator

Sizing a storage tank and sizing a tankless unit are two different problems and this does both. For a tank, enter the showers, baths, laundry and everything else drawn in the busiest hour and it returns the peak hour demand, the recovery rate of the burner or element, the first hour rating you need and the tank that supplies it. For a tankless unit, enter the incoming groundwater temperature and what runs at the same time, and it returns the flow, the temperature rise, the BTU per hour and the electric kilowatts, because a unit that runs two showers in Florida runs one in Minnesota. Hot water figures are built from your own flow rates and the mixing ratio rather than from a fixed table.

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A tank is sized on its first hour rating, not on the gallons painted on the side. Add up the hot water your household draws in its busiest hour and this works back to the tank that covers it.

Stamped on the head. 2.5 gpm is the federal maximum, 1.8 is a low flow head, and older heads run 3 gpm and more.
120 F is the usual setting. Above 130 F scalds a child in seconds.
From the data plate. Watts x 3.412 gives BTU/h, so a 5,500 W element is 18,767 BTU/h.
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A tankless unit never runs out, it runs cold. It is sized on how much water you run at once and how far it has to be heated, and the incoming groundwater temperature is what decides that. Start with the first box.

The number that decides the unit. Roughly 37 to 50 F across the northern states, 50 to 60 F through the middle of the country, and 60 to 77 F in the deep south, Hawaii and coastal California. Run the cold tap in February and measure it if you can.
105 F is a comfortable shower. Set it higher only if a dishwasher without its own heater needs 120 F.
Around 82% for a non-condensing gas unit and 94% for a condensing one. Ignored for the electric figure, where the element is effectively 100%.
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