What size water heater: first hour rating
Count the hot water uses in the household's busiest hour. The total is the peak hour demand, and the first hour rating on the EnergyGuide label should be at least that.
Gallons per use are DOE's averages from its peak-hour worksheet (20 for a shower, 2 for shaving, 3 for hand dishwashing or food prep, 7 for an automatic dishwasher, 25 for a top-loading washer, 15 for a front-loader). They are estimates: change them for long showers, a big tub or water-saving fixtures. Pick the hour of the day (morning, noon or evening) when the household uses the most hot water.
| Use | Gallons | Times | Gallons in the hour |
|---|
The first hour rating is how many gallons of hot water a full tank delivers in an hour. It's on the EnergyGuide label, top left, as “Capacity (first hour rating)”, and in the maker's literature. It depends on tank size and the burner or element, so two 50-gallon tanks can rate differently. This sizes storage and heat pump water heaters; for tankless units use flow rate instead. An estimate: local code and the AHJ govern the installation.
Size a tank water heater by its first hour rating, not its gallons. Using DOE's worksheet, add the hot water used in the busiest hour: about 20 gallons per shower, 2 for shaving, 3 for hand dishwashing, 7 for a dishwasher and 15 to 25 for a clothes washer. Three showers, a shave and dishes come to 65 gallons.
How to use the water heater size calculator
Pick the busiest hour
Morning, noon or evening: when the household uses the most hot water.
Count the uses
Showers, shaving, dishwashing, dishwasher and laundry loads in that hour; change the gallons if needed.
Match the rating
Look for a first hour rating at least equal to the total on the EnergyGuide label.
What the water heater size calculator does
DOE worksheet
The peak-hour uses and average gallons from DOE's water heater sizing guide.
Editable estimates
Change gallons per use for long showers, big tubs or water-saving fixtures.
Label-ready answer
Gives the first hour rating to look for, the number printed on the EnergyGuide label.
Line by line
Each use, its gallons and the total in a table you can show the customer.
- Method
- Peak hour demand = Σ gallons per use × uses in the busiest hour (DOE)
- Gallons per use
- Shower 20, shaving 2, hand dishwashing 3, dishwasher 7, top-loader 25, front-loader 15
- Pick
- First hour rating at least the peak hour demand
- Applies to
- Storage and heat pump water heaters with a tank; tankless units are sized by flow
Water Heater Size Calculator FAQ
What is a first hour rating?
The gallons of hot water a water heater can deliver in an hour, starting with a full tank. It depends on the tank size and the burner or element, and it's printed on the EnergyGuide label as Capacity (first hour rating).
What size water heater for a family of four?
It depends on how they use hot water, not just headcount. If three showers, a shave and hand dishwashing land in the same morning hour, DOE's worksheet gives 65 gallons, so look for a first hour rating of 65 or more.
Is a 50-gallon tank always enough?
Not necessarily. Two 50-gallon heaters can have different first hour ratings because of their burners or elements, so compare the rating, not the tank size.
Are these gallon figures exact?
No. DOE calls them averages from published information. Long showers, rain heads or a soaking tub use more; low-flow fixtures use less. Edit the gallons to match the home.
Does this work for tankless water heaters?
No. Tankless units have no tank to draw down, so they're sized by peak flow in gallons per minute and the temperature rise. Use the tankless water heater size calculator.
Where the numbers come from
Link to this tool
Found it useful? Paste this into your website, newsletter or resource page.
More free tools
All free toolsPrice it right, then book the job
CrewRobin is the AI front office for home-service shops: an AI receptionist that answers every call, qualifies the job and books it, plus missed-call text-back, quotes, invoices and review requests.