Water Heater Sizing Calculator
Work out the first hour rating a tank heater needs, or the flow and heating power a tankless heater needs.
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Sizing Notes
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How to Size a Water Heater
Tank heaters are sized by first hour rating (FHR): how many gallons of hot water the heater can deliver in an hour starting full. The U.S. Department of Energy says to add up your peak hour demand, the hot water you use in your busiest hour, and pick a heater with an FHR at least that large. The FHR is on the yellow EnergyGuide label.
Tankless heaters are sized by flow rate and temperature rise: add up the flow of everything that might run at once, and find how many degrees the heater must raise the incoming water.
500 is 8.33 lb per gallon × 60 minutes. 3,412 BTU per hour make one kilowatt.
Worked Example
DOE Peak Hour Worksheet Values
| Use | Gallons per Use |
|---|---|
| Shower | 20 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-loader | 25 |
| Clothes washer, front-loader (H-axis) | 15 |
From the U.S. Department of Energy's Energy Saver guide to sizing a new water heater. Its tankless example uses 0.75 GPM for a faucet and 2.5 GPM for a shower head.
Tips
Incoming water is much colder in winter in northern states, so size a tankless unit for the coldest month. Gas tankless units need large gas lines and venting; electric ones often need 100 A or more of dedicated circuits. Check the heater's rated flow at your temperature rise on the maker's chart.