Water heater sizing calculator
Tell us what runs in your busiest hour. We use the Department of Energy method to tell you the first hour rating to look for.
Gallons per use and the first hour rating method come from the U.S. Department of Energy, Sizing a New Water Heater. Tankless figures assume incoming water around 50 F and a 120 F setting (a 70 F rise). Colder incoming water needs more capacity. The showerhead flow is the EPA WaterSense standard of 2.5 gallons per minute.
How this calculator works
This follows the U.S. Department of Energy worksheet. You list what runs in your busiest hour, each use is multiplied by the DOE's average hot water per use (20 gallons per shower, 2 per shave, 3 for hand dishwashing, 7 per dishwasher load, 15 or 25 per washer load), and the total is your peak hour demand.
For a tank heater you then look for a first hour rating at least that high. For tankless, you add the flow rates of fixtures running at the same time.
Frequently asked questions
What is a first hour rating?
It is the number of gallons of hot water a tank heater can supply in one hour starting from a full tank. The Department of Energy says to choose a model whose first hour rating at least matches your peak hour demand.
Where do I find the first hour rating?
On the EnergyGuide label, top left, listed as Capacity (first hour rating), or in the manufacturer's literature. Tank size in gallons is not the same number.
How big should a tankless water heater be?
Add the flow rates of the fixtures you will run at once. The Department of Energy lists typical tankless ratings at a 70 degree rise of about 5 gallons per minute for gas units and 2 for electric units.
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Last updated October 2026. Reviewed by the Fix or Hire editorial team. Estimates only, not a quote.