Furnace Airflow Calculator
CFM and temperature rise for a furnace.
Enter your values
ⓘ Live results
Results
- Output capacity
- 64,000 BTU/h
- Airflow
- 1,185 CFM
- CFM per 10,000 BTU output
- 185
- Rise check
- Within typical 30-70°F range
- Airflow at 45°F rise
- 1,317 CFM
Quick answer
Furnace airflow is output BTU divided by 1.08 times the temperature rise, which must fall inside the nameplate range.
CFM = output BTU / (1.08 x rise)
How to use the Furnace Airflow Calculator
- 1Read input rating and efficiency from the furnace nameplate.
- 2Measure supply and return temperatures to get the actual rise.
- 3Adjust blower speed until the rise sits mid-range on the nameplate.
Worked example
Using the values the calculator loads with:
Inputs
- Input rating80000 BTU/h
- Efficiency80 %
- Measured temperature rise50 °F
Results
- Output capacity64,000 BTU/h
- Airflow1,185 CFM
- CFM per 10,000 BTU output185
- Rise checkWithin typical 30-70°F range
- Airflow at 45°F rise1,317 CFM
Frequently asked questions
Rise too high?
Airflow is too low — check the filter, closed registers and duct restriction before touching the blower tap.
Rise too low?
Airflow is excessive, which can cause cold drafts and, on some furnaces, condensation in the heat exchanger.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.