Register Boot Size Calculator
Size a duct boot transition to a register.
Enter your values
Enter the branch duct diameter in in.
Enter the register height in in.
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.
- 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
At a glance
| What it does | Size a duct boot transition to a register. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Branch duct diameter, Register height |
| Main output | Branch duct area |
| Formula | Boot area ≥ branch duct area |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Branch duct diameter(in)
- Enter the branch duct diameter in in.
- Register height(in)
- Enter the register height in in.
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.
What do I need to enter into the Register Boot Size Calculator?
Just 2 values: branch duct diameter, register height. 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 Register Boot Size Calculator work out the 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. It applies the formula Boot area ≥ branch duct area and shows the working so you can check each step by hand.
Is the Register Boot Size 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.