Trunk Duct Size Calculator
Main trunk duct size from total CFM.
Enter your values
Enter the total trunk cfm in CFM.
Enter the design velocity in FPM.
Enter the trunk height in in.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Main trunk duct size from total CFM. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Total trunk CFM, Design velocity, Trunk height |
| Main output | Round equivalent diameter |
| Formula | Area = CFM / velocity |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Total trunk CFM(CFM)
- Enter the total trunk cfm in CFM.
- Design velocity(FPM)
- Enter the design velocity in FPM.
- Trunk height(in)
- Enter the trunk height in in.
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.
What do I need to enter into the Trunk Duct Size Calculator?
Just 3 values: total trunk cfm, design velocity, trunk height. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Trunk Duct Size Calculator work out the answer?
Supply trunk ducts are typically sized for 700-900 FPM velocity to balance airflow capacity against noise and friction. It applies the formula Area = CFM / velocity and shows the working so you can check each step by hand.
Is the Trunk Duct Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.