Water Heater Size Calculator
This water heater size calculator works for both tank and tankless water heaters. In tank mode, you list how often each hot water use happens in your busiest hour, and it totals your peak-hour demand, the minimum first-hour rating to look for. Gallons per use follow the Department of Energy worksheet and can be edited.
In tankless mode, you enter the fixtures that might run at the same time and your desired and incoming water temperatures. The calculator adds up the flow rate in gallons per minute, works out the temperature rise the heater must deliver, and shows the heat output that implies. Switch to metric for liters and degrees Celsius.
Water Heater Size Calculator
How to Use the Water Heater Size Calculator
- 1
Choose tank or tankless
Pick Tank (first-hour rating) for a storage or heat pump water heater, or Tankless (flow and rise) for an on-demand unit. Choose US or metric units.
- 2
Tank: fill in your busiest hour
Enter how many showers, baths, shaves, faucet uses, dishwasher and washer loads happen in your peak hour. Edit the gallons per use if yours differ.
- 3
Tankless: list fixtures running at once
Enter how many showers, faucets, tubs, dishwashers and washers could run at the same moment, then set your desired and incoming water temperatures.
- 4
Read the sizing numbers
Tank mode shows the minimum first-hour rating. Tankless mode shows the flow rate, temperature rise, heat output in BTU/h and equivalent electric kW.
How water heater sizing works
Tank water heaters are sized by first-hour rating (FHR), the number of gallons of hot water the heater can supply in one hour starting with a full tank. It is printed on the EnergyGuide label as "Capacity (first hour rating)". The DOE advice is to choose a model whose FHR at least matches your peak hour demand.
Tankless water heaters are rated by how much they can raise the water temperature at a given flow. You need two numbers: the total flow of fixtures used at once, and the temperature rise from your incoming water to your desired hot water temperature. Without better information, DOE suggests assuming 50°F incoming water and 120°F hot water, a 70°F rise.
Tank: peak hour demand (gal) = sum of (gallons per use x times in the hour); choose FHR >= peak hour demand. Tankless: flow (GPM) = sum of (fixture GPM x number at once); rise (°F) = desired - incoming; heat output (BTU/h) = 500 x GPM x rise; electric kW = BTU/h / 3,412Worked examples
Tank: in your busiest morning hour, three people shower, one person shaves, and someone washes a few dishes by hand. The calculator uses 20 gallons per shower, 2 for shaving and 3 for hand dishwashing.
- Tank: 3 x 20 + 1 x 2 + 1 x 3 = 65 gallons, so look for a first-hour rating of 65 gallons or more
- Tankless: two 2.5 GPM showers plus a 1.5 GPM kitchen faucet = 6.5 GPM
- With 50°F incoming water and 120°F hot water, the rise is 70°F
- Heat output: 500 x 6.5 x 70 = 227,500 BTU/h, equivalent to about 66.7 kW of electric heating
- That is more than many single whole-house gas units deliver, so you would plan on fewer simultaneous fixtures, low-flow fixtures or two units
Hot water per use (DOE worksheet values)
These are the default gallons per use in tank mode. All except the bath come from the Department of Energy Energy Saver peak-hour worksheet; the bath figure is a common estimate. Low-flow showerheads and short showers use less, so adjust them to match your household.
| Use | Hot water per use |
|---|---|
| Shower | 20 gallons |
| Bath (tub fill) | 20 gallons (estimate) |
| Shaving | 2 gallons |
| Hand dishwashing or food prep | 3 gallons |
| Automatic dishwasher | 7 gallons |
| Clothes washer, top-loader | 25 gallons |
| Clothes washer, front-loader (H-axis) | 15 gallons |
How incoming water temperature changes tankless sizing
Incoming water is colder in northern states and in winter, which means a larger temperature rise. The table shows a 3.5 GPM demand (one 2.5 GPM shower plus a 1 GPM bathroom faucet) heated to 120°F, calculated with this tool.
| Incoming water | Rise needed | Heat output | Electric equivalent |
|---|---|---|---|
| 40°F | 80°F | 140,000 BTU/h | 41.0 kW |
| 50°F | 70°F | 122,500 BTU/h | 35.9 kW |
| 60°F | 60°F | 105,000 BTU/h | 30.8 kW |
| 70°F | 50°F | 87,500 BTU/h | 25.6 kW |
Tips, gas vs electric, and when to call a plumber
- Size tankless units for winter. Use your coldest incoming water temperature, not a summer reading.
- Electric tankless units need very large electrical circuits. DOE notes they typically manage a 70°F rise at about 2 GPM, while gas units reach about 5 GPM.
- A gas unit needs a higher input rating than the output shown here because burners are not 100% efficient, plus proper venting and a gas line sized for the load.
- First-hour rating is not tank size. A 50-gallon tank with a strong burner can have a higher FHR than a larger tank with a weak one.
- Water hotter than 120°F raises scald risk, especially for children and older adults.
- Water heater installs involve gas, venting, electrical and pressure relief valves. Have a licensed plumber confirm the size and follow local code.
Features
- Tank mode: peak-hour demand and minimum first-hour rating from DOE gallons-per-use values
- Tankless mode: total flow rate from fixtures running at the same time
- Temperature rise from desired and incoming water temperature
- Heat output in BTU/h and equivalent electric kW for tankless units
- Editable gallons per use and fixture flow rates
- US (gallons, °F) and metric (liters, °C) units
Frequently Asked Questions
What size water heater do I need for a family of 4?
It depends on your busiest hour, not just headcount. If two people shower, the dishwasher runs and a top-loading washer fills in the same hour, that is 2 x 20 + 7 + 25 = 72 gallons with the DOE per-use values, so look for a first-hour rating of at least 72 gallons. Spreading laundry to another time lowers it to 47 gallons.
What is a first-hour rating?
The first-hour rating (FHR) is how many gallons of hot water a tank water heater can deliver in one hour, starting with a full tank of hot water. It depends on tank size and how fast the burner or element reheats. You will find it on the EnergyGuide label. Choose a model with an FHR at least equal to your peak hour demand.
How many GPM tankless water heater do I need?
Add up the flow rates of every fixture that might run at the same time. One 2.5 GPM shower plus a 1 GPM bathroom faucet needs 3.5 GPM. Two showers and a 1.5 GPM kitchen faucet need 6.5 GPM. Then check the model's flow rating at your required temperature rise, because tankless output drops as the rise gets larger.
How do I calculate temperature rise for a tankless water heater?
Subtract your incoming water temperature from the hot water temperature you want. With 50°F incoming water and a 120°F setting, the rise is 70°F. In a cold northern winter with 40°F water, it is 80°F. You can measure incoming temperature by running a cold tap for a minute and holding a thermometer in the stream.
Is a gas or electric tankless water heater better?
Gas units usually deliver more flow. DOE notes gas models typically manage a 70°F rise at about 5 GPM, versus about 2 GPM for electric. Electric units are simpler to vent but can need very large circuits, often more than 100 amps for whole-house models. The right choice depends on your fuel, electrical service and demand, so have a licensed plumber or electrician assess it.
What temperature should I set my water heater to?
120°F (49°C) is the widely recommended setting for most homes because it balances comfort, energy use and scald risk. DOE notes some uses, such as dishwashers without internal heaters, may call for 140°F, which raises the temperature rise to 90°F with 50°F incoming water. Higher settings increase scald risk, so consider anti-scald valves if you go above 120°F.
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