Water Heater Sizing Calculator
Determine the right size tank or tankless water heater based on your household's hot water needs.
How to Size a Water Heater
Tank Water Heaters
Tank water heaters are sized by their First Hour Rating (FHR) - the amount of hot water the heater can supply in the first hour of use. This accounts for both stored hot water and the heating capacity.
- Determine peak hour demand (usually morning or evening)
- Match the FHR to your peak demand
- Common sizes: 30, 40, 50, 65, 80 gallon tanks
Tankless Water Heaters
Tankless (on-demand) heaters are sized by flow rate (GPM) at a specific temperature rise. They heat water instantly as it flows through.
- Calculate total GPM needed for simultaneous fixtures
- Determine required temperature rise (inlet to 120°F output)
- Select unit rated for that GPM at your temperature rise
Temperature Rise
Temperature rise = Desired output temp - Incoming water temp
- Cold climates: 40-50°F inlet → 70-80°F rise needed
- Moderate climates: 50-60°F inlet → 60-70°F rise needed
- Warm climates: 60-70°F inlet → 50-60°F rise needed
BTU Calculation
BTU/hr = GPM × 500 × Temperature Rise
Example: 3 GPM × 500 × 60°F = 90,000 BTU/hr
Example: 3 GPM × 500 × 60°F = 90,000 BTU/hr