Water Heater Size Calculator
Calculate the right water heater capacity for your household. This calculator determines the ideal tank size (in gallons) or tankless flow rate (GPM) based on your family size, bathrooms, and hot water usage patterns.
How to Use This Calculator
- Enter the number of people living in your household
- Input the number of bathrooms (count half baths as 0.5)
- Select whether you have a dishwasher and its usage level
- Indicate if you have a washing machine that uses hot water
- Choose your peak usage pattern based on morning/evening habits
- Click "Calculate Water Heater Size" for recommendations
What is First Hour Rating?
First Hour Rating (FHR) is the most important specification when sizing a tank water heater. It measures how many gallons of hot water a fully heated tank can deliver in one hour. The FHR accounts for both the stored hot water and the heating element's recovery rate. Your water heater's FHR should meet or exceed your household's peak hour demand, which typically occurs during morning showers or evening routines. An undersized FHR means you'll run out of hot water during high-demand periods, while an oversized unit wastes energy keeping excess water hot.
Tank vs. Tankless Water Heaters
- Tank heaters: Store 30-80 gallons, lower upfront cost, simpler installation
- Tankless heaters: Heat on demand, save 20-30% energy, require higher GPM for multiple fixtures
- Heat pump: Most efficient tank option, 2-3x more efficient than electric
- Gas vs Electric: Gas has faster recovery, electric is simpler to install
How Requirements Are Calculated
Peak Demand = (Showers x 10 gal) + (Dishwasher x 6 gal) + (Other uses)
Daily Usage = People x 20 gallons (average)
Tankless GPM = Peak simultaneous fixtures flow rates
Standard hot water usage per fixture:
- Shower: 10 gallons (2.0 GPM x 5 min)
- Bath: 20 gallons per fill
- Dishwasher: 6 gallons per cycle
- Washing machine: 7 gallons (hot water portion)
- Hand washing/shaving: 2 gallons
Sizing Guidelines by Household
- 1-2 people: 30-40 gallon tank or 2-3 GPM tankless
- 3-4 people: 40-50 gallon tank or 3-4 GPM tankless
- 5+ people: 50-80 gallon tank or 5+ GPM tankless