Reducing Trunk Duct Calculator
Step down trunk size after each branch takeoff.
Enter your values
Enter the starting trunk cfm in CFM.
Enter the cfm removed per branch in CFM.
Enter the number of branches you are working with.
Enter the design velocity in FPM.
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.
- After branch 1 (1,200 CFM)
- 16.6 in round
- After branch 2 (1,050 CFM)
- 15.5 in round
- After branch 3 (900 CFM)
- 14.4 in round
- After branch 4 (750 CFM)
- 13.1 in round
- After branch 5 (600 CFM)
- 11.7 in round
- After branch 6 (450 CFM)
- 10.2 in round
Quick answer
A reducing (tapered) trunk keeps velocity roughly constant by shrinking cross-section as branch airflow is removed at each takeoff.
Segment diameter sized for remaining CFM at constant velocity
At a glance
| What it does | Step down trunk size after each branch takeoff. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Starting trunk CFM, CFM removed per branch, Number of branches, Design velocity |
| Main output | After branch 1 (1,200 CFM) |
| Formula | Segment diameter sized for remaining CFM at constant velocity |
| Cost | Free — no sign-up, no download |
How to use the Reducing Trunk Duct Calculator
- 1Enter the trunk's starting CFM and the CFM removed at each branch.
- 2Set the number of branch takeoffs and the target velocity.
- 3Read the reduced trunk diameter needed after each takeoff.
Inputs explained
- Starting trunk CFM(CFM)
- Enter the starting trunk cfm in CFM.
- CFM removed per branch(CFM)
- Enter the cfm removed per branch in CFM.
- Number of branches
- Enter the number of branches you are working with.
- Design velocity(FPM)
- Enter the design velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Starting trunk CFM1200 CFM
- CFM removed per branch150 CFM
- Number of branches6
- Design velocity800 FPM
Results
- After branch 1 (1,200 CFM)16.6 in round
- After branch 2 (1,050 CFM)15.5 in round
- After branch 3 (900 CFM)14.4 in round
- After branch 4 (750 CFM)13.1 in round
- After branch 5 (600 CFM)11.7 in round
- After branch 6 (450 CFM)10.2 in round
Frequently asked questions
Why reduce trunk size at all?
Keeping velocity constant as airflow drops avoids an oversized, noisy-at-low-velocity trunk near the end of the run.
How often should trunk size step down?
Typically after every one or two branch takeoffs, using standard duct size increments.
What do I need to enter into the Reducing Trunk Duct Calculator?
Just 4 values: starting trunk cfm, cfm removed per branch, number of branches, design velocity. 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 Reducing Trunk Duct Calculator work out the answer?
A reducing (tapered) trunk keeps velocity roughly constant by shrinking cross-section as branch airflow is removed at each takeoff. It applies the formula Segment diameter sized for remaining CFM at constant velocity and shows the working so you can check each step by hand.
Is the Reducing Trunk Duct 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.