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ACCA Manual D
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How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact ACCA Manual D reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
Duct sizing matches each duct to the airflow it carries so the system moves the right CFM without excess noise or static pressure. This app uses the equal-friction method from ACCA Manual D, the standard for residential duct design.
How it works
- Start with the airflow (CFM) each run must deliver.
- Pick a target friction rate — commonly around 0.08 to 0.10 in. w.c. per 100 ft for residential systems.
- The app reads the duct diameter that carries that CFM at that friction rate, then gives the equivalent rectangular size if needed.
- It also checks velocity so supply runs stay quiet (roughly 600–900 fpm for branches).
Worked example
A branch carrying 400 CFM at a 0.10 friction rate sizes to about an 8-inch round duct, or roughly a 6 x 10 inch rectangular equivalent.
Double the airflow and the duct does not double in diameter — area scales with the square of the radius, which is exactly why a calculator beats guessing.
Related calculators
Find the required CFM with the airflow CFM calculator after running the heat load calculator.
Duct sizing FAQ
Which method does it use?
The equal-friction method from ACCA Manual D, which sizes every duct to the same friction rate per 100 feet so airflow is balanced across the system.
What friction rate should I use?
Residential systems are commonly designed around 0.08 to 0.10 in. w.c. per 100 ft, derived from the available external static pressure of the air handler and the total effective length of the duct run.
Round and rectangular ducts?
Both. The app sizes round duct first and gives the equivalent rectangular dimensions, so you can fit the duct into the available joist or chase space.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Duct Sizer on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.