Duct Air Velocity Calculator
Calculate air velocity from CFM and actual round or rectangular duct dimensions using the continuity equation.
Actual area
1.069 ft²
Air velocity
935 FPM
Metric velocity
4.75 m/s
Turn the calculation into a complete design workflow
Use the free result as a quick engineering check, then continue into MEPFlow for connected system design, sizing and project calculations.
How the Duct Air Velocity Calculator Works
Air velocity follows directly from continuity. The calculator converts the selected round diameter or rectangular width and height into actual cross-sectional area, then divides airflow by that area.
Use actual duct dimensions for velocity checks. Equivalent round diameter is useful for friction comparison, but the air velocity in a rectangular duct is based on the real rectangular area, not its equivalent-round area.
Velocity limits depend on acoustics, duct location, system type and pressure class. Use the result as a design check rather than treating one FPM value as universally acceptable.
Calculation basis
Velocity = Airflow ÷ Cross-sectional Area
When to Use This Calculator
- Main and branch duct velocity checks
- Existing duct capacity checks
- Acoustic screening
- Round-vs-rectangular comparisons
Engineering Checks Before Final Design
- Use actual internal dimensions.
- Velocity alone does not determine pressure loss.
- Check project acoustic criteria and diffuser/terminal limits.
Frequently Asked Questions
How do I calculate duct velocity from CFM?
Divide airflow in cubic feet per minute by the duct cross-sectional area in square feet.
Should I use equivalent round area for a rectangular duct?
No for velocity. Use the true width × height area of the rectangular section.
Is higher velocity always bad?
No. Acceptable velocity depends on location, noise criteria, pressure class and system economics.
Does this calculate duct friction?
No. Use the MEPFlow Duct Sizer for friction-rate and equivalent-diameter calculations.