How to use the water heater size calculator
- Choose a storage tank or a tankless heater.
- Tank: enter how many times each use happens in your busiest hour of the day.
- Tankless: enter the combined flow rate of fixtures running at the same time, the incoming water temperature and the hot water temperature you want.
Estimate your household's busiest hour honestly; for a tank heater, Decoratoradvice sizes on that peak, not on daily use.
Tank formula: peak-hour demand
The U.S. Department of Energy sizes storage water heaters by their first-hour rating, which should be at least your peak-hour demand. Its worksheet uses these average amounts per use:
| Use | Gallons per use |
|---|---|
| Shower | 20 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-loading | 25 |
| Clothes washer, front-loading (H-axis) | 15 |
Peak-hour demand = sum of (uses × gallons per use)
The first-hour rating is on the EnergyGuide label as "Capacity (first hour rating)".
Tankless formula: flow and temperature rise
Flow needed = sum of fixture flow rates used at the same time
Temperature rise = hot water temperature − incoming water temperature
Heat delivered (BTU/h) = flow (gal/min) × 8.345 × 60 × rise (°F), where 8.345 is the weight of a gallon of water in pounds
The DOE's example: a 0.75 gal/min faucet and a 2.5 gal/min shower together need 3.25 gal/min. Unless you know otherwise, it suggests assuming incoming water at 50 °F; for 120 °F hot water that is a 70 °F rise.
The DOE notes that a 70 °F rise is typically possible at about 5 gal/min for gas-fired units and 2 gal/min for electric units.
Worked examples
A storage tank for a morning with two showers, one shave and one round of hand dishwashing:
2 × 20 + 2 + 3 = 45 gallons of peak-hour demand
Choose a first-hour rating of at least 45 gallons
Add a top-loading wash in the same hour and the demand rises to 70 gallons.
A tankless heater for a 2.5 gal/min shower and a 0.75 gal/min faucet at once, 50 °F inlet, 120 °F outlet:
Flow: 3.25 gal/min
Rise: 70 °F
Heat: 3.25 × 8.345 × 60 × 70 ≈ 113,900 BTU/h (about 33.4 kW) delivered to the water
Gas heaters need a higher input rating than this to deliver it, depending on their efficiency.
Tips
- Pick your realistic busiest hour, such as weekday mornings, rather than adding every possible use.
- Low-flow showerheads and faucets cut both peak demand and tankless flow.
- Electric water heaters can be a large load on a generator; see the generator size calculator.
- Compare running costs with the electricity cost calculator, and upgrade payback with the energy upgrade payback calculator.
Common mistakes
- Sizing a tank by its gallons alone. The first-hour rating is what matters for busy hours.
- Ignoring winter inlet temperatures for tankless units. Colder water needs a larger rise.
- Setting very high temperatures. The DOE notes scalding risk rises at 140 °F.
Frequently asked questions
What size water heater do I need for a family of four?
It depends on your busiest hour, not the number of people. Four showers in one hour alone is 80 gallons of peak demand under the DOE worksheet.
What is a first-hour rating?
The number of gallons of hot water a storage heater can supply in one hour, starting with a full tank.
What size tankless water heater do I need?
One rated for your combined flow at your temperature rise, for example 3.25 gal/min at a 70 °F rise.
What incoming water temperature should I use?
The DOE suggests assuming 50 °F (10 °C) unless you know otherwise. It is colder in winter in many places.
Is tankless better than a tank?
They suit different households. Tankless units heat on demand but are limited by flow; tanks store hot water for peak use.
Key takeaways
- Tank: first-hour rating ≥ peak-hour demand (DOE worksheet).
- Tankless: size for combined flow at your temperature rise.
- Assume 50 °F incoming water unless you know better.
- Gas units need input above the heat delivered to the water.