Tell us how many people live in the house and how many hot-water uses run in the same busy hour. You get the first-hour rating to look for, a tank size and the flow a tankless unit would need.
The numbers are planning estimates. A plumber confirms the right size for your home and the fuel it uses.
We start with the hot water your household uses in its busiest hour. We count about 12 gallons for each person, plus about 9 gallons for each hot-water use you count in that hour, such as a dishwasher, a washer or a second shower. The total is rounded to the nearest 5 gallons. That is the first-hour rating.
A tank is matched to the first standard size that covers it. Tanks come in 30, 40, 50, 65 and 80 gallons. A tankless unit works on flow instead, so we count about 2 gallons per minute for each use running at once, with a floor of 3 and a ceiling of 11.
The first-hour rating is the number to match to your home. It is how much hot water a heater can deliver in one busy hour, and it is printed on the label of every tank. A tank can have a first-hour rating higher than its size, because it heats water while you use it.
The tank size is the standard size that covers your busy hour, so treat it as a starting point for the conversation with a plumber. If your busy hour is more than the largest standard tank is sized for, the result says so. That is the point to look at a high-recovery heater, two tanks or a tankless setup.
A bigger tank is not always better. A tank larger than you need keeps more water hot around the clock. The aim is a rating that fits your busy hour.
Long showers, a soaking tub, a laundry day with a hot wash and a house full of guests all push the busy hour up. Gas heaters recover faster than electric ones, so a gas tank usually has a higher first-hour rating than an electric tank of the same size. Always check the rating on the label of the model you are looking at.
Four people with two hot-water uses in the busy hour come to a first-hour rating of about 65 gallons, which points to a 65 gallon tank. Set the numbers for your own home in the calculator, since a second bathroom or a big laundry load changes the answer.
It is the amount of hot water a water heater can deliver in one hour, starting with a full tank. It counts the water stored in the tank and the water the heater warms while you draw. It is the best single number for matching a heater to a household.
No. A larger tank keeps more water hot all day and night, even when nobody is home. Match the rating to your busy hour and no more.
Yes. A gas heater warms water faster than an electric one, so the same tank size usually has a higher first-hour rating on gas. A plumber reads the label on the specific model to size it properly.
A tankless unit makes hot water as you use it, so there is no tank to size. It is sized by flow and by how cold the incoming water is. The tankless flow calculator on this site works through both.
The figures are planning rules of 12 gallons per person and 9 gallons per hot-water use counted in the busiest hour, matched to standard 30 to 80 gallon tank sizes. They are estimates, not label ratings.
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