Tankless Water Heater Sizing & BTU Calculator
Calculate required tankless water heater flow rate (GPM), groundwater temperature rise (ΔT), natural gas BTU/hr capacity, and electric kW breaker specs.
Calculated Results
Frequently Asked Questions
How do you size a tankless water heater?
Sizing requires two numbers: (1) Flow Rate in GPM (add up all hot water fixtures you plan to run simultaneously), and (2) Temperature Rise ΔT (subtract incoming winter groundwater temperature from your target 120°F output).
Why does inlet water temperature dramatically impact tankless sizing?
In northern US states with 40°F winter groundwater, heating water to 120°F requires an 80°F rise. In Florida with 70°F water, only a 50°F rise is required. The same unit produces nearly double the hot water GPM in the south compared to the north.
Can whole-house electric tankless heaters replace gas heaters?
Electric tankless heaters require immense electrical capacity. A unit delivering 5 to 6 GPM with a 65°F rise demands 28 to 36 kW of power, drawing 120 to 150 Amps on dedicated 240V circuits—often requiring a costly 300A or 400A electrical service panel upgrade.
What size gas line is required for a 199,000 BTU tankless heater?
Most 199k BTU gas tankless water heaters require a dedicated 3/4-inch black iron or CSST gas pipe. Standard 1/2-inch lines cannot supply enough volume unless the gas meter is located less than 10 to 15 feet away.