Enter the airflow in cubic feet per minute and select the duct cross-sectional area into the calculator to determine the air velocity in feet per second.
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CFM to Ft/s Formula
The following formula is used to calculate the air velocity in feet per second from airflow in cubic feet per minute and duct area.
FTS = (CFM / 60) / A
Variables:
- FTS is the air velocity in feet per second
- CFM is the airflow in cubic feet per minute
- A is the duct cross-sectional area in square feet
To calculate feet per second, first divide the airflow in CFM by 60 to convert it to cubic feet per second, then divide by the duct area in square feet.
What is CFM to Ft/s Conversion?
CFM to ft/s conversion is the process of converting volumetric airflow into linear air velocity. CFM tells you how much air moves each minute, while ft/s tells you how fast that air is traveling through a duct. Because velocity depends on the size of the opening the air passes through, the duct cross-sectional area must be known to perform the conversion accurately.
How to Calculate CFM to Ft/s?
The following steps outline how to calculate feet per second from CFM.
- First, determine the airflow in cubic feet per minute (CFM).
- Next, determine the duct cross-sectional area in square feet (A).
- Convert CFM to cubic feet per second by dividing by 60.
- Finally, divide that result by the duct area to calculate the air velocity in feet per second.
- After inserting the values and calculating the result, check your answer with the calculator above.
Example Problem:
Use the following variables as an example problem to test your knowledge.
Airflow (CFM) = 300
Duct Area (A) = 1.0 ft²
Feet Per Second (FTS) = (300 / 60) / 1.0 = 5 ft/s
CFM to Ft/s Conversion Table
The table below shows a few common airflow and duct area combinations using the formula above.
| Airflow (CFM) | Duct Area (ft²) | Velocity (ft/s) |
|---|---|---|
| 60 | 1.0 | 1 |
| 120 | 1.0 | 2 |
| 300 | 1.0 | 5 |
| 600 | 1.0 | 10 |
| 300 | 0.5 | 10 |
| 300 | 2.0 | 2.5 |
| 900 | 1.5 | 10 |
In HVAC design and troubleshooting, airflow is often rated in CFM while airspeed is discussed in ft/s or ft/min. Understanding how to convert between these values makes it easier to evaluate duct sizing, compare airflow targets, and check whether a system is operating as expected.