What size water heater do I need? Sizing calculator
Reviewed 4 primary sourcesSee sources
For a tank water heater, the number that matters is the first-hour rating: how much hot water it can deliver in a busy hour. For a tankless unit, it is the flow rate it can deliver at your temperature rise. Both methods here come from U.S. Department of Energy and ENERGY STAR guidance.
Storage tank: first-hour rating
- Use the first-hour rating on the label, not the tank size, to compare storage water heaters.[1]
- Estimate your household’s busiest hour of hot water use by adding up what happens in that hour, using DOE’s average gallons per activity.[1]
- ENERGY STAR measures first-hour rating with water delivered at 125 °F.[2]
DOE’s guidance is to choose a model whose first-hour rating is within about 2 gallons of your peak-hour demand. Its worked example (two showers, two shampoos, one shave, two hand-and-face washes and one meal prepared) comes to 49 gallons.[1]
Per-use gallons: [1]
Tankless: flow rate and temperature rise
- List the hot water uses that run at the same time in your home and add up their flow rates.[3]
- Work out the temperature rise: your hot water setting (about 120 °F) minus the temperature of the cold water entering your home.[3]
- Choose a model that delivers that flow at that rise, using the manufacturer’s flow-versus-rise chart. ENERGY STAR rates tankless flow over a 67 °F rise.[2]
DOE’s Solution Center example adds a shower, a clothes washer, a kitchen sink and a dishwasher to get 4.5 gallons per minute, and needs that flow at a 68 °F rise for its Seattle location.[3]
What this calculator doesn’t cover
- DOE’s per-activity gallons are averages, credited to the Gas Appliance Manufacturers Association, and assume no water-saving fixtures. The fact sheet dates from 2001.[1]
- The tankless flow figures are a sample home’s draws; the shower figure is the hot-water share of a mixed shower, and the example’s inlet temperature is specific to Seattle.[3]
- Some small uses, like hand washing, can need less flow than a tankless unit needs to switch on; check the model’s minimum flow.[3]
- Colder climates generally need a higher-capacity tankless unit; ENERGY STAR suggests talking to an experienced plumber.[4]
Sources
Facts on this page are restated in our own words from the manufacturer and government documents below. Formats and codes change between product lines, so check the document for your exact model.
- Technology Fact Sheet: Water Heating (Building America)U.S. Department of Energy (prepared by NAHB Research Center, Southface Energy Institute, ORNL, NREL) · DOE Building America fact sheet 26465 (PDF dated 2001) · Page 2, 'Storage Tank Water Heater Selection' text and 'First Hour Rating/Peak Hour Demands' table · retrieved
- Water Heater Key Product CriteriaENERGY STAR (U.S. EPA) · Criteria tables and 'Glossary' · retrieved
- Gas-Fired Tankless Water Heaters (Building America Solution Center)Pacific Northwest National Laboratory for U.S. DOE · Description tab, 'Equipment Sizing' section and Table 2 'Hot Water Draws and Maximum Flow Rate for a Sample Home' (image) · retrieved
- Technical Bulletin: Demand Water Heaters (ENERGY STAR New Homes)ENERGY STAR (U.S. EPA) · 'Capacity and Peak Demand' and 'ENERGY STAR Certified Demand Water Heaters (Gas only)' sections (PDF 2018) · retrieved
Primary publishers: U.S. Department of Energy (prepared by NAHB Research Center, Southface Energy Institute, ORNL, NREL), ENERGY STAR (U.S. EPA), Pacific Northwest National Laboratory for U.S. DOE. Spotted an error? Email the address in the footer with the model and the document you are using.