What size water heater do I need?
Size a tank by your busiest hour of hot-water use, and a tankless by the fixtures you run at once plus how cold your incoming water is. Both methods are below, done the way the industry does them.
Water heater size calculator
tank FHR · tankless GPMCount the hot-water uses that happen in your busiest hour, usually the morning. The total is the First Hour Rating to shop for.
Count the fixtures that run at the same time, not the total in the house.
Add the uses in your busiest hour to see the First Hour Rating to shop for.
On the sources: the federal consumer water-heating pages were taken down in 2025, so these figures are reconstructed from AHRI, an EPRI report hosted by ENERGY STAR, an EPA groundwater map, and manufacturer guides. The temperature-rise method is well corroborated; the gallons and GPM tables are moderate-confidence planning numbers. Nothing you enter leaves your browser.
First Hour Rating, not household count
A storage tank is sized by its First Hour Rating: how many gallons of hot water it can deliver during a busy first hour, combining the tank's stored volume with how fast it reheats. That figure is on the yellow EnergyGuide label, and it is the number to match, not the raw tank size.
To find your target, add up the hot-water uses in your busiest hour. A shower is about 20 gallons, a hand wash about 4, the dishwasher about 14. The industry worksheet's own example, three showers plus a shave, a shampoo, and a hand dishwash, comes to 70 gallons. You then shop for a unit whose First Hour Rating meets or beats your number.
Flow rate at your temperature rise
A tankless heater never runs out, but it can only heat so much water so fast. You size it two ways at once: the flow rate in gallons per minute of the fixtures you run together, and the temperature rise it has to provide, which is your target temperature (usually 120F) minus the incoming water temperature.
That second half is the step thin tools skip. Incoming groundwater runs near 40F in the far north and near 77F in the deep south, and a tankless unit delivers less flow the bigger the rise it has to make. A unit that gives 5 GPM in Florida might give barely 3 in Minnesota. Size to your region's water, not a headline maximum.
Common questions
What size water heater do I need for a family of 4?
Size to your busiest hour, not your household count. A family of four often peaks with two or three back-to-back showers plus the dishwasher or washer, which lands around 60 to 80 gallons of demand in that hour. That is the First Hour Rating to look for on the label. Build your actual peak hour in the tank calculator to get a number for your routine.
What size tankless water heater for 2 bathrooms?
Count the fixtures you would run at the same time. Two showers at once is about 5 GPM; add a sink or the kitchen and it climbs. Then find your region's incoming water temperature, because a tankless unit delivers less flow the colder the water starts. A 2-bathroom home in a cold climate needs more unit than the same home in Florida.
What is First Hour Rating?
First Hour Rating, or FHR, is how many gallons of hot water a tank can deliver in a busy first hour, combining what is stored with how fast it reheats. It is printed on the yellow EnergyGuide label. Matching FHR to your peak-hour demand matters more than tank size alone, since a smaller tank with fast recovery can out-deliver a larger slow one.
Why does my region matter for a tankless heater?
A tankless heater is rated by how much it can raise the water temperature at a given flow. Incoming groundwater is near 40F in the far north and near 77F in the deep south, so the same unit has to work much harder in Minnesota than in Florida. A tankless sized off a warm-climate GPM number is undersized up north. That is why the calculator asks for your region.