What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Airflow: 400 cfm.
- Target velocity: 700 ft/min.
- Rectangular duct height: 8 in.
What happens, step by step
- 400 cfm of air flows in at 700 feet per minute.
- Free area is airflow divided by speed: 0.571 sq ft of opening.
- That fits a 10.24 in round duct, or 8 in × 10.29 in rectangular.
The answer: round duct diameter (minimum) is 10.24 in.
How it’s calculated
- Area (ft²) = CFM ÷ velocity (FPM)
- Round diameter = √(4 × area ÷ π)
- Rectangular width = area ÷ height
Worked example
400 CFM at 700 FPM, rectangular option 8 in tall.
Inputs
- Airflow
- 400 cfm
- Target velocity
- 700 ft/min
- Rectangular duct height
- 8 in
Results
- Round duct diameter (minimum)
- 10.24 in
- Rectangular duct width at that height
- 10.29 in
- Free area
- 0.571 sq ft
Assumptions and limits
- Equal-area sizing only; it does not account for friction loss, fittings or blower static pressure.
- A rectangular duct with the same area as a round duct has more friction; ACCA Manual D uses equivalent-diameter methods.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
What size duct for 400 CFM?
At 700 FPM, about 10.2 in diameter, so a 10 in round duct runs slightly faster and a 12 in slower. Friction-rate design may choose differently.
Why is my duct noisy?
High velocity causes noise. Branch and register velocities much above 900 FPM are often audible.