Volume airflow equals the free cross-sectional area multiplied by average air speed through that area. Use the open area air can pass through, and keep the speed and area units attached to their values.
The relationship is Q = A × V, where Q is volume airflow, A is free area, and V is average air speed through that area. Square feet multiplied by feet per minute gives cubic feet per minute (CFM).
The example uses 2 ft² × 600 ft/min = 1200 CFM. Nominal outside duct dimensions do not necessarily describe the free opening. Area and air speed must refer to the same section of the airflow.
Find duct airflow
Multiply duct free area by average air speed. Label both values with their units.
Start a new calculation
- Press Clear entry, then Clear entry.C→C
Select Area
- Press Second, then Select area units.2nd→Area
Enter free area
Free area
Press 2, then Square feet, then Multiply.2→ft²→×
Select Air Speed
- Press Second, then Select air speed units.2nd→Air Speed
Enter average speed
Average air speed
Press 600, then feet per minute, then Equals.600→FPM→=
- Example result1200 CFM
Keep in mind
Use the open area air can pass through and the average speed across it. To find speed instead, divide airflow by area. Use 2nd when a unit is above its button.
Keep the guide beside your calculation.
These examples are built into Figure. Open Help while you calculate, then return to the same guide and reading position.
See Help in Figure →