Reducing Trunk Duct Calculator
Step down trunk size after each branch takeoff.
Enter your values
ⓘ Live results
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
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
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.
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.
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.