Home / Calculators / Water heater size (first-hour rating)
Calculator
Water heater size (first-hour rating)
Size a tank water heater by your busiest hour of hot water use.
Your numbers
Results
How this works
Following DOE guidance, add up hot water used in your peak hour: shower about 10 gal, shaving 2, hand dishwashing or food prep 4, dishwasher 6, clothes washer 7. Choose a heater with a first-hour rating (FHR) within 1 to 2 gallons of that number.
Sources
Related guide
Water heaters & water treatment guide
Compare hot-water systems and water-treatment options by household demand, fuel, water quality, installation needs, and ongoing costs.