Duct design
Duct Size Calculator
Calculate a preliminary round duct diameter from assigned CFM and a project-selected target velocity.
Calculate round duct diameter
Assign airflow to the run first. The chosen velocity must have a documented project basis.
How to use this calculator
- Assign the CFM required for the specific duct run.
- Enter a target velocity selected for the project; this tool does not prescribe one.
- Use the theoretical diameter to select a practical available size, then recheck actual velocity and resistance.
Formula
Round diameter (in) = 12 × √(4 × area ÷ π)
Variables
| Input | Meaning | Limit |
|---|---|---|
| CFM | Assigned air volume flow for one run | Must come from a documented system basis |
| FPM | Chosen face velocity | Not a universal velocity recommendation |
| Diameter | Theoretical round internal diameter | Does not include fittings or available sizes |
Example
At 800 CFM and 700 FPM, required area is 800 ÷ 700 = 1.143 ft². The theoretical round diameter is about 14.5 inches. Choose an available duct size and calculate its actual velocity before proceeding.
Result interpretation
This tool uses only the CFM/area/velocity relationship. Final duct design also needs the complete pressure path: straight length, fittings, transitions, dampers, terminals, filters, coils, fan performance, sound, balancing, construction, and applicable requirements.
Continue the duct check
Use the Duct Velocity Calculator after choosing a size, then review the straight-run calculation with the Duct Friction Loss Calculator. Start with the CFM Calculator if the airflow basis is not yet defined.
Frequently asked questions
Is a lower velocity always better?
No. Lower velocity can require more space and cost. Acceptable velocity depends on the system, acoustic objective, pressure budget, and design method.
Does this tool select a standard duct size?
No. It returns a theoretical size. Select an available duct and recheck its actual area, velocity, fittings, and pressure losses.
Can this replace final HVAC duct design?
No. It is a preliminary geometry check only and does not model the complete system resistance or distribution.