Water heater sizing calculator
Size a storage tank from your peak hour of hot water use, or a tankless unit from the flow you need and how cold the incoming water is.
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Result
Your peak hour, use by use
| Activity | Gallons each | Times | Gallons |
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Sizing a storage tank
Tank water heaters are matched by first hour rating, not by the number of gallons printed on the side. The first hour rating is how much hot water the unit can deliver in one hour starting full, and it includes what the burner or element reheats while you are drawing. A 50 gallon gas tank often has a first hour rating around 70 gallons.
The method, from the Department of Energy's sizing guidance, is to work out your peak hour demand and then buy a unit whose first hour rating meets it:
- Pick the busiest hour in a normal day, usually a weekday morning.
- Count what happens in that hour: showers, a dishwasher load, a wash cycle.
- Add up the gallons. Match or beat that figure with the first hour rating on the label.
Oversizing a tank is not free. A larger tank loses more heat through its walls all day, every day, whether you use the water or not.
Sizing a tankless unit
A tankless heater never runs out, so the question is not capacity but how much it can heat at once. Two things decide it: the flow you want in gallons per minute, and how far the water has to be raised in temperature.
The constant 500 carries water's specific heat and density together with the 60 minutes in an hour. A unit delivering 4 gpm at a 70 °F rise therefore needs about 140,000 BTU/h.
Incoming water temperature is what catches people out. The same unit that gives 6 gpm in Florida may give only 3 gpm in Minnesota, because the water arrives 30 °F colder and the unit has to work twice as hard per gallon. This is why a tankless heater that performed well at a friend's house in a warm state may disappoint further north.
Questions
What size water heater do I need for a family of 4?
Work from the busiest hour rather than the household size. A typical family of four taking two morning showers with a dishwasher running has a peak hour demand near 50 gallons, which points to a tank with a first hour rating of 50 or more, usually a 40 to 50 gallon unit.
Is first hour rating the same as tank size?
No, and this is the most common mistake. First hour rating includes the water already in the tank plus what the unit reheats during that hour, so it is usually larger than the tank. Always compare the first hour rating with your peak demand.
What size tankless water heater do I need?
It depends on the flow you want and your incoming water temperature. Two showers at once is about 4 gpm; at a 70 °F rise that needs roughly 140,000 BTU/h, which is a large residential gas unit. The same flow at a 40 °F rise needs only about 80,000.
Why does my tankless unit run cold in winter?
Groundwater is colder in winter, so the temperature rise is larger and the unit's maximum flow drops. A heater rated 6 gpm at a 45 °F rise may manage only 4 gpm at 70 °F. Sizing for summer water is the usual cause.
Can I replace a 50 gallon tank with electric tankless?
Often not without an electrical upgrade. Matching a 50 gallon tank in a cold climate can mean 27 kW or more, which is roughly 112 A at 240 V before the 125% continuous factor. Check your service capacity first.