Water Heater Size Calculator for Tank and Tankless

Estimate the size you need

Count what could run in your busiest hour of hot water use, usually the morning.

Showers Baths Shaving or hand washing Dishwasher loads Washing dishes by hand Laundry loads (hot)

Estimates use the U.S. Department of Energy peak-hour method and typical fixture flow rates. Confirm with the unit's first-hour rating or flow chart before you buy.

Tank water heaters are sized by first-hour rating, the amount of hot water they can deliver in an hour starting with a full tank. Tankless heaters are sized by flow rate at the temperature rise your climate needs.

Tank size alone can mislead. Two 50 gallon tanks can have very different first-hour ratings, so compare the rating printed on the label.

How the calculator works

For tanks, it adds up typical hot water use per activity during your busiest hour, following the U.S. Department of Energy peak-hour method. For tankless, it adds the flow of fixtures running at once and pairs it with the temperature rise for your region.

How to choose a tank size

Size a tank by its first-hour rating (FHR), printed on the yellow EnergyGuide label, not by gallons alone. FHR is how many gallons of hot water the heater can deliver in one busy hour starting with a full tank. Add up what your household uses in its busiest hour, such as back-to-back showers, a dishwasher and a load of laundry, and pick a heater whose FHR meets or beats that number. The calculator above does that math for you.

Fuel changes the answer. Gas burners reheat water faster than electric elements, so a gas tank often has a higher FHR than an electric tank of the same size. That is why a household that is happy with a 40 gallon gas heater may need a larger electric or heat pump model.

40 vs 50 gallon water heater

	40 gallon	50 gallon
Best for	Smaller households, lighter peak use	Larger households, overlapping showers and appliances
Space	Fits tighter closets	Taller or wider; check clearances
Cost	Slightly lower to buy and keep hot	Slightly higher
What to compare	First-hour rating on the label	First-hour rating on the label

Rheem's rule of thumb puts households of three to four people at about 50 to 65 gallons, but real use varies. If you often run out of hot water now, see our hot water runs out fast guide before simply buying a bigger tank; sediment or a failed dip tube can cut capacity. For installed prices by type, see our water heater replacement cost guide.

Common questions

What size water heater do I need?

Choose a heater whose first-hour rating meets your busiest hour of hot water use. The calculator above estimates that number for your household.

Is a 40 or 50 gallon water heater better?

A 50 gallon suits larger households and overlapping use; a 40 gallon fits smaller homes and tight spaces. Compare first-hour ratings, not just tank size.

What is a first-hour rating?

The number of gallons of hot water a tank heater can deliver in one hour starting full. It is printed on the EnergyGuide label.

Water heater size calculator