What this calculator does
Find the right size water heater so you don't run out of hot water at the busiest time of day. A heater that's too small runs out in the middle of the morning rush. One that's too large costs more to buy and to keep hot. The right size depends on how much hot water your household uses at its busiest, not on the number of bedrooms. This calculator follows the U.S. Department of Energy's (DOE's) sizing method for both kinds of heater.
For a storage tank, count the showers, dishwasher loads, laundry loads and other uses in your busiest hour. The calculator totals them using DOE's average gallons per use. That total, the peak hour demand, is the smallest first hour rating (FHR) to look for. The FHR is how much hot water a tank can deliver in one hour. For a tankless heater, add up the flow of the fixtures that run at the same time. Then find the temperature rise, from your incoming water to the temperature you want. The calculator also estimates the heat output in BTU per hour (a unit of heat). It shows the power in kilowatts an electric model would need, too.
Where it's used
Why this math matters outside the classroom:
- Replacing a water heater. A family counts 2 showers, 1 dishwashing session and 1 laundry load in the morning: 40 + 3 + 15 = 58 gallons.
- Choosing tankless. A homeowner adds a 2.5 gpm shower and a 0.75 gpm faucet to get the 3.25 gpm a tankless unit must supply.
- Temperature rise. A plumber in a cold climate subtracts 45 °F incoming water from a 120 °F setting to size for a 75 °F rise.
- Electric supply. An electrician estimates 500 × 3.25 × 70 = 113,750 BTU/h, about 33.3 kW, for an electric tankless unit.
- Science of heat. Students see that warming 8.33 lb of water by 70 °F takes about 583 BTU, since 1 BTU warms 1 lb by 1 °F.
How to use it
- Choose a tank or tankless heater.
- For a tank, enter how many of each hot water use happen in your household's busiest hour.
- For tankless, enter the showers and faucets that run at once, their flow rates, and the incoming and wanted water temperatures.
- Compare the result with the first hour rating, or the flow at that temperature rise, listed for the models you're considering.
What the inputs mean
- Showers in the busiest hour
- Think of the single hour when the most hot water is used. That's often early morning, when several people shower back to back. DOE's worksheet counts about 20 gallons of hot water per shower.
- Shaves (running water) in the busiest hour
- About 2 gallons of hot water each.
- Hand dishwashing or food prep in the busiest hour
- About 3 gallons of hot water each.
- Automatic dishwasher loads in the busiest hour
- About 7 gallons of hot water each.
- Top-loading clothes washer loads in the busiest hour
- About 25 gallons of hot water each.
- Front-loading clothes washer loads in the busiest hour
- About 15 gallons of hot water each.
- Flow per shower head
- Most shower heads sold in the US are rated at 2.5 gallons per minute (gpm) or less. Many low-flow heads use 1.5 to 2.0. To measure yours, catch the flow in a bucket for one minute.
- Other hot water flow at the same time (optional)
- For example a dishwasher or washing machine filling.
- Incoming cold water temperature
- Incoming water temperature changes with climate and season. It's lowest in winter in cold regions. DOE suggests 50 °F if you don't know. A thermometer under a cold tap gives a better figure, and sizing for winter is safer.
Formula and method
peak hour demand (gal) = Σ (uses in the busiest hour × gallons per use) gallons per use (DOE worksheet): shower 20, shave 2, hand dishwashing or food prep 3, automatic dishwasher 7, top-loading washer 25, front-loading washer 15 minimum first hour rating = peak hour demand tankless flow (gpm) = showers × shower gpm + faucets × faucet gpm + other temperature rise = wanted temperature − incoming temperature heat output (BTU/h) ≈ 500 × gpm × rise (°F); kW = BTU/h ÷ 3,412.14
Σ means "add up". gpm is gallons per minute, and the rise is how many degrees the water must warm up. BTU/h is British thermal units of heat per hour, and kW is kilowatts of electric power.
A storage heater's first hour rating is how many gallons of hot water it can supply in an hour, starting with a full tank. It depends on the tank size and on how fast the burner or element can reheat. DOE advises choosing a model whose FHR at least matches the household's peak hour demand.
A tankless heater has no stored supply, so it must heat the water as fast as it flows. Its rating is a flow rate at a given temperature rise. The power needed comes from how much heat water takes to warm up. One BTU warms one pound of water by 1 °F, and a gallon of water weighs about 8.33 pounds. So each gallon per minute raised 1 °F takes about 8.33 × 60 ≈ 500 BTU per hour.
Worked example
A household's busiest hour has two showers, one sink of hand dishwashing and one front-loading washer load.
- Showers: 2 × 20 = 40 gallons.
- Hand dishwashing: 1 × 3 = 3 gallons.
- Front-loading washer: 1 × 15 = 15 gallons.
- Peak hour demand: 40 + 3 + 15 = 58 gallons.
Result: Look for a tank water heater with a first hour rating of at least 58 gallons.
Understanding the result
The first hour rating is printed on the EnergyGuide label of new storage water heaters as "Capacity (first hour rating)". It's usually higher than the tank's stated size, because the heater keeps reheating as you use water. Two models with the same tank size can have quite different ratings.
For tankless units, compare your flow and temperature rise with the maker's chart. DOE notes that a typical gas tankless unit gives about a 70 °F rise at 5 gallons per minute. An electric one gives that rise at about 2 gallons per minute. Electric whole-house units also need a large electrical supply. So check the kilowatt figure against your electrical panel with an electrician.
Assumptions and limitations
- Gallons per use are DOE's averages. Long showers or large tubs use more.
- The tankless heat output is the heat delivered to the water. A gas unit's input rating is higher, depending on its efficiency.
- The busiest hour is the right basis for a storage tank. Daily totals aren't used.
- It doesn't cover commercial uses, or large soaking tubs, which can need more hot water than the tank holds at once.
- Heat pump water heaters may reheat more slowly than electric resistance or gas models of the same size. Check the FHR of the model itself.
- Local codes, venting, gas line size and electrical capacity can limit what you can install. A licensed plumber should confirm the final choice.
Frequently asked questions
What size tankless water heater do I need for two showers?
Two 2.5 gpm showers need 5 gallons per minute. Heating 50 °F incoming water to 120 °F is a 70 °F rise. That takes about 500 × 5 × 70 = 175,000 BTU per hour of heat, or roughly 51 kW for an electric unit.
Is first hour rating the same as tank size?
No. Tank size is how much water the tank holds. The first hour rating is how much hot water it can deliver in an hour from full, including what it reheats. A 50-gallon tank commonly has a first hour rating above 50 gallons.
What temperature should a water heater be set to?
DOE suggests 120 °F for most household use. It notes that 140 °F raises the risk of scalding; that setting is used for some dishwashers without their own heaters. Check your appliances' instructions.
Why size for winter?
Incoming water is coldest in winter, so the heater needs a bigger temperature rise. A tankless unit sized for summer water may deliver lukewarm water in January. A storage tank reheats more slowly then, too.
Sources
- U.S. Department of Energy, Energy Saver: Sizing a New Water Heater (archived 1 January 2026) (checked 2026-10-19)
How this calculator is checked
Method: U.S. DOE Energy Saver 'Sizing a New Water Heater': peak hour demand worksheet (gallons per use) and the FHR ≥ peak demand rule; tankless sizing by simultaneous flow and temperature rise (50 °F default inlet). Heat output by the standard 500 × gpm × ΔT°F rule (8.33 lb/gal × 60 min/h) and 1 kWh = 3,412.14 BTU. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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