Water Heater Size Calculator
Count how often each use happens in your household's busiest hour of the day, usually the morning. You'll get the first-hour rating a tank needs and, from the uses that run at the same moment, the flow a tankless heater needs.
- AHRI sizing worksheet
- DOE Building America method
- Tank and tankless
2 What size to buy
Updates as you type| Showers | 3 × 20 gallons = 60 gallons |
|---|---|
| Shaving | 1 × 2 gallons = 2 gallons |
| Hair shampoo | 1 × 4 gallons = 4 gallons |
| Hand dishwashing | 1 × 4 gallons = 4 gallons |
| Peak-hour demand | 70 gallons: buy a tank whose first-hour rating is at least this |
| 1 shower | 1.5 gpm |
| Clothes washer | 2 gpm |
| 1 sink | 0.5 gpm |
| Dishwasher | 0.5 gpm |
| Temperature rise | 120°F hot water − 50°F incoming water = 70°F |
The first-hour rating is on the yellow EnergyGuide label of a tank water heater, and a tankless model's label lists its maximum flow in gallons per minute. Check the tankless model's flow at your temperature rise in its specifications, since its flow drops as the rise gets bigger.
Set the thermostat to 120°F (49°C), as DOE and the Consumer Product Safety Commission recommend. CPSC warns that 140°F (60°C) water can burn in six seconds.
Quick answer
Size a tank water heater by its first-hour rating: add up the hot water your household uses in its busiest hour, using AHRI's figures of about 20 gallons per shower, and buy a tank rated at least that. Three showers, a shave, a shampoo and hand dishwashing come to 70 gallons. Size a tankless heater by flow instead: a shower, clothes washer, sink and dishwasher at once need about 4.5 gallons per minute at the temperature rise for your incoming water.
Peak-hour demand for common mornings
| Busiest hour | Peak-hour demand | Federal draw pattern |
|---|---|---|
| 1 shower, hand and face washing | 24 gallons | low |
| 2 showers, 2 washings, a shampoo | 52 gallons | medium |
| 3 showers, shave, washing, shampoo | 70 gallons | medium |
| 4 showers, shave, 2 washings, shampoo | 94 gallons | high |
| 3 showers, shave, washing, shampoo, dishwasher | 84 gallons | high |
AHRI gallons per use: shower 20, shave 2, hand and face washing 4, shampoo 4, hand dishwashing 4, automatic dishwasher 14, clothes washer 32. Choose a first-hour rating at least this high. Use the calculator above for your exact size.
Tank water heaters: first-hour rating
A tank heater's first-hour rating (FHR) is how many gallons of hot water it can supply in an hour, starting full of hot water. It is printed on the yellow EnergyGuide label, and it matters more than the tank size, because a tank with a strong burner or element recovers faster. DOE's Building America Solution Center recommends a tank with a first-hour rating equal to or greater than your peak-hour demand, using AHRI's industry worksheet:
| Use | Gallons of hot water each time |
|---|---|
| Showers | 20 |
| Baths | 20 |
| Shaving | 2 |
| Hand and face washing | 4 |
| Hair shampoo | 4 |
| Hand dishwashing | 4 |
| Automatic dishwasher | 14 |
| Food preparation | 5 |
| Automatic clothes washer | 32 |
AHRI's figures assume no water-saving fixtures. DOE's Building America tool recommends sizing with full figures like these rather than low-flow ones, so the heater keeps up for its whole life, which it puts at 13 years on average.
Worked example
A household's busiest morning hour has 3 showers (3 × 20 = 60 gallons), a shave (2), a shampoo (4) and hand dishwashing (4): a peak-hour demand of 70 gallons, the same example AHRI uses. Look for a tank with a first-hour rating of 70 gallons or more.
Tankless water heaters: flow and temperature rise
A tankless heater has no stored hot water, so it is sized by the flow of everything running at the same moment, and by how much it must warm the incoming water. DOE's Building America guide gives an example of hot water flows at about 120°F: a shower 1.5 gpm, a clothes washer 2, a kitchen sink 0.5 and a dishwasher 0.5, a total of 4.5 gallons per minute.
The temperature rise is your hot water setting minus the incoming water temperature. EPA's map of average shallow ground water temperatures runs from about 41°F along the northern border to about 77°F in South Florida, so the rise to 120°F ranges from about 79°F to 43°F. DOE's guide uses Seattle, with ground water near 52°F, as its example: a 68°F rise. Water is usually colder in winter, so size for that.
| 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 |
Manufacturers list a model's flow at several temperature rises, and the flow falls as the rise grows. The federal rating on the label is measured over a 67°F rise.
Which type?
DOE's Energy Saver guide says gas tankless heaters can be 24% to 34% more efficient than storage tanks for homes that use 41 gallons or less of hot water a day, and 8% to 14% more efficient for homes that use around 86 gallons a day, and that a home using hot water in several places at once may need two or more tankless units. It says most storage heaters last 10 to 15 years and tankless heaters can last more than 20.
Set it to 120°F
DOE and the Consumer Product Safety Commission recommend setting a water heater to 120°F. CPSC warns that most adults suffer third-degree burns from 150°F water in two seconds, that burns also occur in six seconds at 140°F and thirty seconds at 130°F, and that children under five and older adults are most at risk.
Frequently asked questions
What size water heater do I need for a family of 4?
Size it by your busiest hour rather than by headcount. Using AHRI's figures, four showers alone use about 80 gallons in an hour, so a family that showers back to back needs a tank with a first-hour rating of about 80 gallons or more. A family that spreads showers out needs less.
What is a first-hour rating?
The gallons of hot water a tank water heater can supply in an hour, starting fully heated. It is on the yellow EnergyGuide label, and federal rules measure it at a 125°F outlet temperature.
How many gpm tankless water heater do I need?
Add the flow of the hot water uses that run at the same time. DOE's Building America example of a shower, clothes washer, kitchen sink and dishwasher together comes to 4.5 gallons per minute, at the temperature rise for your incoming water.
What temperature should a water heater be set to?
120°F, as DOE and the Consumer Product Safety Commission recommend, to save energy and prevent scalds.
Last reviewed 2026-11-07. See how we calculate for our sources and assumptions.