Register Boot Size Calculator
Size a duct boot transition to a register.
Enter your values
ⓘ Live results
Results
- Branch duct area
- 28.3 sq in
- Minimum boot width at height
- 7.1 in
- Recommended boot size
- 8 x 4 in
Quick answer
A boot's throat area should be at least equal to the branch duct's cross-sectional area to avoid restricting airflow at the register.
Boot area ≥ branch duct area
How to use the Register Boot Size Calculator
- 1Enter the branch duct diameter feeding the boot.
- 2Choose the register height available (floor registers are often 2.25-4 in tall).
- 3Ensure the boot's width x height area is at least the duct's cross-sectional area.
Worked example
Using the values the calculator loads with:
Inputs
- Branch duct diameter6 in
- Register height4 in
Results
- Branch duct area28.3 sq in
- Minimum boot width at height7.1 in
- Recommended boot size8 x 4 in
Frequently asked questions
What happens if the boot is undersized?
It chokes airflow right at the register, raising noise and cutting delivered CFM even if the duct upstream is correctly sized.
Straight or offset boot?
Offset boots let the duct enter above or beside the register but add turbulence — keep runs short into the boot.
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.