Trunk Duct Size Calculator
Main trunk duct size from total CFM.
Enter your values
ⓘ Live results
Results
- Round equivalent diameter
- 19.1 in
- Rectangular size at height
- 28.8 x 10 in
- Cross-section area
- 2 sq ft
- Velocity check
- 800 FPM
Quick answer
Supply trunk ducts are typically sized for 700-900 FPM velocity to balance airflow capacity against noise and friction.
Area = CFM / velocity
How to use the Trunk Duct Size Calculator
- 1Total the CFM of all branches the trunk must feed.
- 2Pick a trunk velocity — 700-900 FPM is typical for residential main trunks.
- 3Choose an available trunk height and solve for the matching width.
Worked example
Using the values the calculator loads with:
Inputs
- Total trunk CFM1600 CFM
- Design velocity800 FPM
- Trunk height10 in
Results
- Round equivalent diameter19.1 in
- Rectangular size at height28.8 x 10 in
- Cross-section area2 sq ft
- Velocity check800 FPM
Frequently asked questions
Why not just use branch sizing rules?
Trunks carry the sum of many branches, so they need larger cross-section even though the velocity target is similar.
Should trunk size taper?
Yes, reducing trunk size as branches peel off keeps velocity consistent along the run.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.