Water Heater Size Calculator — What Size Water Heater Do I Need?
Find the right water heater size for your household. Get recommendations for tank, tankless, and heat pump water heaters based on household size and usage patterns.
How to Use This Calculator
- Enter the number of people in your household and bathrooms
- Select your typical hot water usage pattern
- Choose the type of water heater you're considering
- Select your climate for accurate tankless sizing
- Click "Calculate Water Heater Size" for a specific recommendation
How the Calculation Works
// Tank water heater sizing:
peak_gallons = people × gallons_per_person × usage_multiplier
// Low: 12 gal/person | Medium: 18 gal/person | High: 24 gal/person
// Required FHR = peak_gallons (match First Hour Rating)
// Tankless water heater sizing:
gpm_needed = simultaneous_fixtures × avg_gpm_per_fixture
// Shower: 2.0 GPM | Kitchen: 1.5 GPM | Dishwasher: 1.5 GPM | Washer: 1.5 GPM
temp_rise = target_temp - inlet_temp
// Cold climate: inlet = 40°F | Moderate: 55°F | Warm: 70°F
btu_needed = gpm × temp_rise × 500 // for gas
kw_needed = gpm × temp_rise × 2.5 // for electric Tank heaters are sized by First Hour Rating (FHR) — how many gallons can be delivered in the first hour at peak demand. For tankless heaters, you need to calculate the maximum flow rate (GPM) and temperature rise from cold inlet water to desired hot water temperature. The BTU or kW rating required increases with colder inlet water.
Variables:
- peak_gallons: Estimated peak hour hot water demand in gallons
- FHR: First Hour Rating — gallons of hot water in the first hour
- gpm_needed: Maximum flow rate needed for simultaneous fixtures (tankless)
- temp_rise: Temperature rise needed (target temp minus cold inlet water temp)
- btu_needed: BTU/h required for gas tankless heater
Pro Tips
- Match the water heater's First Hour Rating (FHR) to your peak morning demand — not just tank size
- Tankless heaters save 24–34% energy vs tank heaters for low-use households
- In cold climates, ground water temperature can be as low as 40°F — tankless heaters need higher BTU capacity
- Heat pump water heaters are 2–3× more efficient than electric resistance but need 700+ cubic feet of space
- A 50-gallon gas water heater is sufficient for 3–4 people in most climates
Recommended Products
Frequently Asked Questions
What size water heater do I need for a family of 4?
For 4 people: a 50-gallon gas tank or 65-gallon electric tank. For tankless: a 180,000 BTU gas unit or 18–24 kW electric unit in moderate climates. Gas tankless is generally preferred for larger families due to faster recovery.
What is First Hour Rating and why does it matter?
First Hour Rating (FHR) measures how many gallons a tank heater can deliver in the first hour of use starting with a full hot tank. It accounts for both tank capacity and heating element recovery. Match your FHR to your peak morning demand — a larger tank with a weak element may underperform a smaller tank with a powerful burner.
Is a tankless water heater worth it?
Tankless heaters save 24–34% energy for average households and provide endless hot water. Upfront cost is 2–3× higher than tank heaters, but they last 5–10 years longer (20+ years vs 10–12 years). Best value for households using more than 41 gallons daily or who want the energy savings over time.
What temperature should a water heater be set to?
120°F is the standard recommended temperature — hot enough to kill most bacteria, cool enough to prevent most scalding. 140°F is recommended if anyone in the household has a compromised immune system. Set tankless units at 120°F; reduce storage tank temperature if children are present.
Can I install a water heater myself?
In many jurisdictions, homeowners can replace like-for-like water heaters with a permit. New installations, fuel conversions, and electrical panel work typically require a licensed plumber. Gas connections should always be tested for leaks after installation.