Duct Size Calculator
Size a round or rectangular air duct from airflow and target velocity.
1,200 FPM (6.1 m/s)
Result
Duct diameter
12.36 in
313.96 mm
Airflow
1,699.01 m³/h
Usage examples
Finding a round duct diameter for a given airflow
Enter the required airflow (CFM) and a target velocity to get the appropriate round duct diameter right away.
Sizing a rectangular duct to fit a ceiling space
When one side of a rectangular duct is constrained, calculate the other side needed to meet the required airflow.
Designing quickly with typical velocities by section
Pick a typical recommended velocity for supply mains, branches, or return ducts to speed up design work.
Frequently asked questions
What are the velocity presets based on?
They reflect typical recommended velocity ranges commonly cited by SMACNA and similar guidelines. Adjust them to match your actual design criteria.
What does equivalent diameter mean?
It's the diameter of a round duct that would have the same friction performance as the rectangular duct, using the standard ASHRAE/SMACNA formula.
Does this only support CFM?
Input is in CFM (ft³/min); the result also shows an m³/h conversion for reference.
Does this account for insulation or duct liner thickness?
No. This calculator computes only the clear air-flow cross-section; any increase in actual outer dimensions from insulation or liner needs to be accounted for separately.