Water Heater Size Calculator
Tank or tankless size for your home.
Enter your values
ⓘ Live results
Results
- Recommended tank size
- 70 gallons
- Tankless flow needed
- 4.4 gpm
- Required heating capacity
- 154,000 BTU/h
- First hour rating target
- 112 gallons
- Peak demand
- 264 gallons/hour
Quick answer
Tank heaters are sized on first hour rating and household size; tankless units are sized on peak simultaneous flow.
How to use the Water Heater Size Calculator
- 1Enter household size and bathroom count.
- 2Estimate how many fixtures run at once.
- 3Set the temperature rise for your climate.
Worked example
Using the values the calculator loads with:
Inputs
- Household size4
- Bathrooms2
- Peak simultaneous fixtures2
- Flow per fixture2.2 gpm
- Temperature rise70 °F
Results
- Recommended tank size70 gallons
- Tankless flow needed4.4 gpm
- Required heating capacity154,000 BTU/h
- First hour rating target112 gallons
- Peak demand264 gallons/hour
Frequently asked questions
Tank or tankless?
Tankless suits steady demand and saves standby losses; tanks handle short heavy peaks better.
What is first hour rating?
The gallons a tank can deliver in one hour starting fully hot — the number that really matters.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.