Calculators · Water Heater Size Calculator
PlumbingWater Heater Size Calculator
Work out the first hour rating a storage tank needs, or the flow and temperature rise a tankless unit needs.
Peak hour demand
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How to size a storage tank water heater
The U.S. Department of Energy sizes a storage water heater, including a heat pump water heater with a tank, with its first hour rating. That is the number of gallons of hot water the heater can supply in one hour, starting with a tank full of hot water. ENERGY STAR defines it as an estimate of the maximum volume of hot water a storage water heater can supply in an hour that begins with the heater fully heated, measured at a 125°F outlet temperature. You then look for a model whose first hour rating at least matches your peak hour demand, the most hot water your home uses in a single hour.
| Use | Average gallons per use |
|---|---|
| Shower | 20 |
| Shaving (0.05 gallon per minute) | 2 |
| Hand dishwashing or food prep (2 gallons per minute) | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-loader | 25 |
| Clothes washer, H-axis | 15 |
These are the averages in the Department of Energy worksheet, from its Sizing a New Water Heater page. We read an archived copy dated January 1, 2026, because the live page returned “not found” when we checked in October 2026. The department says the numbers are averages taken from information published on websites. The calculator lets you change the gallons per use, for example for a low-flow showerhead. The ENERGY STAR definition is in its key product criteria for residential water heaters.
Example: the Department of Energy peak hour
The department’s own example is a household that uses 3 showers, 1 shave and 1 round of hand dishwashing in its busiest hour.
| Use | Operation | Gallons in 1 hour |
|---|---|---|
| Showers | 3 × 20 | 60 |
| Shaving | 1 × 2 | 2 |
| Hand dishwashing | 1 × 3 | 3 |
| Peak hour demand | 65 |
The department’s page prints 66 gallons as the total of this example, but its three lines add up to 65 (60 + 2 + 3). We follow the lines, so the calculator shows 65. Either way, this household would look for a model with a first hour rating of about 65 gallons or more.
How to size a tankless water heater
A tankless or demand-type water heater is rated by the highest temperature rise it can deliver at a given flow rate. The Department of Energy gives three steps: list the hot water fixtures you expect to use at the same time, add their flow rates in gallons per minute, and work out the temperature rise. If you do not know a flow rate, the department says to hold a pan or bucket under the tap and measure the flow for a minute.
| Step | Formula |
|---|---|
| 1. Flow needed (gpm) | flow of fixture 1 + flow of fixture 2 + … |
| 2. Temperature rise (°F) | temperature you want − incoming water temperature |
The department’s example runs a hot water faucet at 0.75 gallons per minute and a showerhead at 2.5 gallons per minute together, for a total of 3.25 gallons per minute. With incoming water at 50°F and a tap temperature of 120°F, the rise is 70°F. It also says the flow through a demand water heater should be at least 3.25 gallons per minute. Typically a 70°F rise is possible at 5 gallons per minute through gas-fired demand water heaters and at 2 gallons per minute through electric ones. Compare those numbers with the rating on the spec sheet of the model you want, because the typical figures are not a guarantee for any one unit.
For a dishwasher without an internal heater the department mentions 140°F, which needs a rise of 90°F in that same example. It warns that a temperature of 140°F increases the chance of scalding. Faster flow rates or colder incoming water can also lower the temperature at the farthest faucet.
Tips for using the result
- The Department of Energy says to find the time of day (morning, noon or evening) when your home uses the most hot water, and to keep in mind how many people live there. Then count the uses in that hour only.
- Check the first hour rating on the EnergyGuide label, or in the manufacturer’s literature if the model is a heat pump water heater.
- For a tankless unit, count every fixture that could run at once, even if it rarely happens.
- For a combination water and space heating system, the Department of Energy says to consult a qualified contractor.
- Use the same temperature rise when you compare the rated flow of a tankless unit with the result.
- To turn the wattage on a heater’s label into amps, use the watts to amps calculator. To size a generator for your appliances, use the generator size calculator.
Frequently asked questions
How do I calculate how big of a water heater I need?
For a tank, add up the hot water your household uses in its busiest hour. The Department of Energy worksheet multiplies the gallons per use by how many times each use happens in that hour. Then look for a model whose first hour rating is at least that number. For a tankless unit, add the flow rates of the fixtures that run at the same time and work out the temperature rise.
Is a 50 gallon water heater enough for a family of 5?
The tank size alone cannot answer that. The first hour rating depends on the tank capacity, on the source of heat and on the size of the burner or element, according to the Department of Energy. Two 50 gallon models can have different ratings. Work out your peak hour demand with the calculator and compare it with the rating printed on the model you are considering.
Is a 40 gallon water heater enough for a family of 6?
It depends on how much hot water the family uses in its busiest hour and on the first hour rating of that 40 gallon model. Six people showering in the same hour need far more than the same six spread over a morning. Enter your own busiest hour in the calculator and check the result against the rating on the label.
What size tankless water heater do I need to replace a 40 gallon tank?
A tankless unit has no tank, so the old tank size does not matter. The Department of Energy says tankless units are rated by the maximum temperature rise at a given flow rate. Add the flow of the fixtures that will run together, subtract the incoming water temperature from the temperature you want, and compare both numbers with the rating of the unit.
What temperature rise does a tankless water heater need?
Subtract the incoming water temperature from the temperature you want at the tap. Unless you know the real incoming temperature, the Department of Energy assumes 50°F. For most uses it assumes 120°F, which is a rise of 70°F. For a dishwasher without an internal heater it mentions 140°F, a rise of 90°F, and warns that 140°F raises the chance of scalding.
Where do I find the first hour rating of a water heater?
The EnergyGuide label lists it in the top left corner as “Capacity (first hour rating)”. The Federal Trade Commission requires that label on new conventional storage water heaters but not on heat pump water heaters. The manufacturer’s literature may also give the rating.
Does a heat pump water heater with a tank use the same method?
Yes. The Department of Energy says to size storage water heaters, including a heat pump water heater with a tank, with the first hour rating. Combination water and space heating systems are different, and the Department of Energy says to consult a qualified contractor for those.
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