Branch Duct Size Calculator
Branch duct size from register CFM.
Enter your values
ⓘ Live results
Results
- Round branch diameter
- 5.5 in
- Flex duct recommended size
- 6 in nominal
- Cross-section area
- 0.167 sq ft
Quick answer
Branch ducts feeding a single register are typically sized for 500-700 FPM to keep noise and friction low.
Diameter = sqrt(4 x CFM / (velocity x pi)) x 12
How to use the Branch Duct Size Calculator
- 1Enter the CFM the branch must deliver to its register.
- 2Use 500-600 FPM for quiet rooms, up to 700 FPM elsewhere.
- 3Round up to the nearest available flex or rigid duct size.
Worked example
Using the values the calculator loads with:
Inputs
- Branch CFM100 CFM
- Design velocity600 FPM
Results
- Round branch diameter5.5 in
- Flex duct recommended size6 in nominal
- Cross-section area0.167 sq ft
Frequently asked questions
What size is typical for a bedroom register?
Most 80-120 CFM branches use 6 inch round duct.
Should I round up or down?
Always round up — undersized branches raise velocity, noise and pressure drop.
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.