Altitude Airflow Correction Calculator
Correct CFM and capacity for altitude.
Enter your values
Enter the sea-level airflow in CFM.
Enter the altitude in m.
Enter the air temperature in °C.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Correct CFM and capacity for altitude. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Sea-level airflow, Altitude, Air temperature |
| Main output | Density ratio |
| Formula | corrected CFM = sea-level CFM / density ratio |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Sea-level airflow(CFM)
- Enter the sea-level airflow in CFM.
- Altitude(m)
- Enter the altitude in m.
- Air temperature(°C)
- Enter the air temperature in °C.
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.
What do I need to enter into the Altitude Airflow Correction Calculator?
Just 3 values: sea-level airflow, altitude, air temperature. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Altitude Airflow Correction Calculator work out the answer?
Air thins with altitude, so a system needs proportionally more CFM to deliver the same heating or cooling capacity. It applies the formula corrected CFM = sea-level CFM / density ratio and shows the working so you can check each step by hand.
Is the Altitude Airflow Correction Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.