Altitude Airflow Correction Calculator
Correct CFM and capacity for altitude.
Enter your values
ⓘ Live results
Results
- Density ratio
- 0.835
- Corrected airflow
- 479 CFM
- Extra airflow needed
- 79 CFM
- Capacity derate if CFM unchanged
- 16.5%
Quick answer
Air thins with altitude, so a system needs proportionally more CFM to deliver the same heating or cooling capacity.
corrected CFM = sea-level CFM / density ratio
How to use the Altitude Airflow Correction Calculator
- 1Enter the airflow a sea-level design calls for.
- 2Add your site altitude and design temperature.
- 3Increase fan speed or duct size to hit the corrected CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Sea-level airflow400 CFM
- Altitude1500 m
- Air temperature20 °C
Results
- Density ratio0.835
- Corrected airflow479 CFM
- Extra airflow needed79 CFM
- Capacity derate if CFM unchanged16.5%
Frequently asked questions
When should I bother?
Above about 750 m the derate passes 8% and starts to matter for sizing.
Does it affect gas furnaces too?
Yes — most need a burner derate of roughly 4% per 300 m.
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.