Duct Takeoff Size Calculator
Size a branch takeoff fitting from a trunk.
Enter your values
ⓘ Live results
Results
- Calculated diameter
- 5.2 in
- Nominal takeoff size
- 6 in round
- Recommended fitting
- Standard round takeoff
Quick answer
Takeoff fittings are sized to match the branch duct diameter feeding a register, typically 4 to 8 inches for residential branches.
Takeoff diameter = branch duct diameter
How to use the Duct Takeoff Size Calculator
- 1Enter the branch CFM the takeoff must supply.
- 2Size for the same velocity used for the branch duct itself.
- 3Round up to the nearest standard takeoff size, using a conical fitting for larger branches to cut turbulence.
Worked example
Using the values the calculator loads with:
Inputs
- Branch CFM90 CFM
- Design velocity600 FPM
Results
- Calculated diameter5.2 in
- Nominal takeoff size6 in round
- Recommended fittingStandard round takeoff
Frequently asked questions
Does takeoff style matter?
Yes — straight-cut 90-degree takeoffs create far more turbulence loss than conical or bellmouth takeoffs.
Should the takeoff match the branch exactly?
Yes, a takeoff smaller than its branch duct becomes the bottleneck for that 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.