Quick answer
A tank water heater is sized by its first-hour rating: the gallons it can supply in your busiest hour. A tankless heater is sized by flow: the gallons per minute of everything that runs at once, at the temperature rise from your incoming water to 120°F. DOE's Building America example of a shower, clothes washer, kitchen sink and dishwasher at once needs 4.5 gallons per minute.
How a tankless heater is sized
DOE's Building America Solution Center sizes a tankless heater by the practical maximum flow needed at any one time. List the hot water uses that can happen together and add their flows. Its example, in gallons per minute of hot water at about 120°F:
| Use | Hot water flow |
|---|---|
| Shower | 1.5 gpm |
| Clothes washer | 2 gpm |
| Kitchen sink | 0.5 gpm |
| Dishwasher | 0.5 gpm |
| Total | 4.5 gpm |
The guide notes a showerhead may be rated higher, such as 2 gpm, but only part of that is hot water mixed with cold. Federal rules cap showerheads at 2.5 gpm and bathroom and kitchen faucets at 2.2 gpm, and WaterSense-labeled showerheads use no more than 2.0 gpm.
The temperature rise
The heater must warm incoming water to your hot water setting, typically about 120°F. The difference is the temperature rise, and a heater's flow drops as the rise grows. EPA's map of average shallow ground water temperatures runs from about 41°F in the north to 77°F in South Florida:
| Incoming water | Rise to 120°F |
|---|---|
| 41°F | 79°F |
| 50°F | 70°F |
| 59°F | 61°F |
| 68°F | 52°F |
| 77°F | 43°F |
DOE's guide uses Seattle, where ground water averages about 52°F: a 68°F rise, so it would look for a model that delivers at least 4.5 gpm at a 68°F rise. Size for winter, when incoming water is usually coldest. The federal maximum flow rating on a tankless label is measured over a 67°F rise, so a colder climate can need a larger model than the label suggests.
How a tank heater is sized
A tank is sized by its first-hour rating, the hot water it can supply in an hour starting full. Add up your busiest hour with AHRI's worksheet, about 20 gallons per shower, and choose a rating at least that high. See what size water heater you need, or use the water heater size calculator, which works out both.
How they compare
- Efficiency: DOE's Energy Saver guide says gas tankless heaters can be 24% to 34% more efficient than storage tanks in homes using 41 gallons or less of hot water a day, and 8% to 14% more efficient in homes using around 86 gallons a day.
- Several uses at once: DOE says a home that uses hot water in several places at the same time may need two or more tankless units.
- Life: DOE says most storage heaters last 10 to 15 years, while tankless heaters can last more than 20.
- Burner size: DOE's Building America guide says gas storage tanks often have 30,000 to 50,000 Btu/h burners and tankless heaters 120,000 to 200,000 Btu/h, which can mean a larger gas line.
- Small draws: the same guide notes that flows below 0.5 gpm are often wanted but not all tankless models can run that low.
ENERGY STAR-certified gas tankless heaters must deliver at least 2.8 gpm over a 67°F rise with a Uniform Energy Factor of 0.95 or more.
Frequently asked questions
What size tankless water heater do I need?
Add the flows of the hot water uses that run at the same time and check the model's flow at your temperature rise. DOE's example of a shower, clothes washer, kitchen sink and dishwasher together needs 4.5 gpm, at a 68°F rise in Seattle.
How many gpm do I need for two showers?
At DOE's example of 1.5 gpm of hot water per shower, two showers need 3 gpm, plus anything else running at the same time, at your temperature rise.
Is tankless better than a tank?
DOE's Energy Saver guide says gas tankless heaters can be 24% to 34% more efficient in homes using 41 gallons or less of hot water a day, and can last more than 20 years. They keep supplying hot water, but only up to their flow rate at your temperature rise, so they need enough capacity for everything that runs at once, and often a larger gas line.
What temperature rise should I use?
Your hot water setting, typically 120°F, minus your incoming water temperature in winter. EPA's ground water map ranges from about 41°F in the north to 77°F in South Florida.
Calculators for this
Water Heater Size
Find what size water heater you need: the first-hour rating for a tank from your busiest hour of hot water use, or the flow and temperature rise for a tankless.
Heating Cost Comparison
Compare the cost of heat from natural gas, propane, heating oil, electric heat and a heat pump, per million Btu and for a whole winter, with your own prices.
Space Heater Size
What size space heater do you need?
Published 2026-11-07. Sources: DOE Building America Solution Center, Gas-fired tankless water heaters; EPA, Average shallow ground water temperatures; DOE, Energy Saver Guide (2022); ENERGY STAR, Residential water heaters key product criteria; 10 CFR 430.32 (showerhead and faucet flow limits); EPA WaterSense, Showerheads; AHRI, Worksheet for Estimating Peak Hour Demand/First Hour Rating. See how we calculate.