Duct Velocity Calculator
Enter your duct airflow (CFM) and duct dimensions to calculate the air velocity in feet per minute. Compare the result against recommended velocity ranges for residential and commercial systems.
Inputs
Velocity Formula
Round duct:
v = Q / (π × (d/2 ÷ 12)²)Rectangular duct:
v = Q / ((W × H) / 144)- v = velocity (ft/min)
- Q = airflow (CFM)
- d = round duct inside diameter (inches)
- W, H = rectangular duct width and height (inches)
- ÷12 converts inches to feet for the area calculation; ÷144 converts in² to ft²
Recommended Velocity Ranges
| Duct type | Low (ft/min) | Target (ft/min) | Max (ft/min) |
|---|---|---|---|
| Residential branch supply | 400 | 600–700 | 900 |
| Residential trunk supply | 500 | 800–1000 | 1200 |
| Residential return | 400 | 600–700 | 800 |
| Return grille face | 200 | 300–500 | 600 |
| Commercial supply trunk | 600 | 1000–1500 | 2000 |
Source: ACCA Manual D residential duct design guidelines and ASHRAE Fundamentals (as of June 2026). Commercial applications vary — consult local codes and ASHRAE Standard 62.1.
Related Tools
- Duct sizing calculator — CFM to correct duct size
- Duct sizing chart — quick reference table
- CFM calculator — room dimensions to CFM
- Return air duct sizing
- Flex duct sizing chart
Frequently Asked Questions
ACCA Manual D residential guidelines: branch ducts 600–700 ft/min, trunk ducts 800–1000 ft/min. Commercial systems can run 1000–2000 ft/min in main trunks. High velocity causes noise, turbulence, and premature wear on duct liners and fittings.
Use a manometer and pitot tube, or an anemometer (vane-type or hot-wire). Measure at multiple points across the duct cross-section and average them (traverse method). For a rough field check, compare measured static pressure to your design static pressure.
Turbulence increases sharply above about 800 ft/min in branch ducts. The turbulent boundary layer creates broadband noise that transmits through the duct wall into the room. Fittings (elbows, tees, transitions) amplify this effect at high velocity.
Below 400 ft/min in cooling mode, you may experience supply air stratification — cold air settling near the floor instead of mixing with room air. This reduces effective cooling comfort even if the required CFM is being delivered.