How to read the result
The calculator adds the flow of each fixture you selected, subtracts the assumed inlet temperature from a 105 °F setpoint to get the temperature rise, and multiplies flow × rise × 500 to give the heat output in BTU per hour (8.34 lb per gallon × 60 minutes). Dividing by 3,412 gives the same output in kW. For gas units the listing states an input, not an output, so the result also shows the input a unit of roughly 82 % (non-condensing) or 95 % (condensing) thermal efficiency would need; those bands are planning assumptions, not a measured figure for any listed unit.
Compare the required GPM at your rise with the maximum flow a listing states. Sellers usually print the maximum at a modest rise, so a unit that says 9.5 GPM may deliver far less at a 63 °F rise. The rise you need — not the brochure GPM — is what the installer sizes against.
Before choosing a heater
- Decide which fixtures genuinely run together on a winter morning.
- Find your winter inlet temperature from your utility or a local plumber.
- Read the stated maximum flow, fuel and input on the exact listing.
- Ask a licensed installer whether your gas line, venting route or electrical panel can serve that unit under local code.
This tool answers an arithmetic question and nothing else. It does not establish that a unit is safe or permitted in your house: gas piping, venting, combustion air, breakers and conductors are decided by a licensed plumber, gas fitter or electrician, and we give no installation guidance.
Read our research methodology and the sizing guide on GPM and temperature rise.