Water Heater Sizing Calculator (First Hour Rating and Tankless GPM)
Count the showers, baths, loads and washes that happen in your busiest hour and the calculator adds them up with the Department of Energy worksheet figures to give the first hour rating (FHR) a tank water heater needs, with typical tank sizes for your household. The second half sizes a tankless heater: the gallons per minute of fixtures running together and the temperature rise from your incoming water to 120 °F.
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Tank heaters: first hour rating
The DOE Energy Saver worksheet assigns gallons of hot water to each use: shower 20, bath 20, shaving 2, hand and face washing 2, automatic dishwasher 6, hand dishwashing 4, clothes washer 7. Add up everything that happens in the busiest hour and the total is the first hour rating (FHR) the heater must have. FHR is printed on the EnergyGuide label; it combines the tank volume with the recovery rate, which is why a 40 gallon gas heater can have a higher FHR than a 50 gallon electric one. Pick a heater whose FHR is within a few gallons of your number.
Tankless heaters: flow and temperature rise
Add the flow of the fixtures that run at the same time (shower 2.5 gpm, faucet 1.5, dishwasher 1.5, clothes washer 2.0) and subtract the incoming water temperature from the set point to get the rise. A tankless heater is rated at a given gpm for a given rise, and its output falls as the rise grows, so use the coldest incoming water of the year. The heat needed is gpm × 500 × rise in BTU per hour (divide by 3,412 for kilowatts); a gas unit's burner input is that figure divided by its efficiency.
Worked example
In the busiest hour a family takes 3 showers (60 gal), one person shaves (2), two wash hands and faces (4) and the dishwasher runs once (6): 72 gallons, so look for a heater with a first hour rating of about 72. For three people that is typically a 40 to 50 gallon gas or electric tank. For a tankless heater, one shower and one faucet at the same time is 4.0 gpm; from 50 °F incoming water to 120 °F is a 70 °F rise, which takes 4.0 × 500 × 70 = 140,000 BTU per hour of heat output, about 41 kW for an electric unit.
Frequently asked questions
What is first hour rating?
The gallons of hot water a full tank heater can deliver in its first hour of use, printed on the yellow EnergyGuide label. It depends on the tank size, the burner or element size and the set point, so compare heaters on FHR rather than gallons.
Is 120 degrees the right set point?
The DOE recommends 120 °F for most homes to limit scalding and energy use. Dishwashers without a booster heater, and some health-care or dairy uses, call for higher storage temperatures with a mixing valve at the fixtures.
Why does a tankless heater need so many BTUs?
A tank heater stores heat over hours; a tankless unit must make it instantly. Raising 4 gpm by 70 °F takes 140,000 BTU per hour, which is why whole-house gas tankless units are rated at 150,000 to 199,000 BTU per hour and electric ones draw 18 to 36 kW on multiple 240 V circuits.
Can I use smaller numbers for low-flow fixtures?
Yes. A 1.8 gpm showerhead and 1.2 gpm faucets reduce both the first hour draw and the tankless flow. The worksheet figures above are conservative averages for standard fixtures.
Do these sizes satisfy the plumbing code?
The code requires an approved water heater of adequate capacity but leaves the sizing to the designer, so the household table is a typical guide, not a rule. Gas supply, venting, relief valve discharge and expansion tanks are code items; check the adopted plumbing and fuel gas codes and the manufacturer's instructions.
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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.