Duct Sizing Calculator

Most residential and light commercial duct design uses the equal friction method: pick a friction rate, then size every run so it loses that same pressure per 100 feet. Designing to a friction rate keeps the system balanced without re-computing the whole trunk every time a branch changes.

This calculator solves for the diameter that produces your target friction rate at your airflow, then rounds up to the nearest standard size — so you land slightly under your design pressure drop rather than over it.

• 0.08 in. w.c. per 100 ft — a common residential supply design point.
• 0.05 in. w.c. per 100 ft — typical for return ductwork, where you want less resistance.
• 0.10 in. w.c. per 100 ft — commercial work, or long runs where you trade a little noise for a smaller duct.

Lower friction rates mean bigger ducts and quieter air. Higher rates mean tighter ductwork but more static pressure for the blower to work against — and static pressure is what you're fighting when you get an airflow complaint.

Above roughly 1,200 fpm you'll hear it — and a noise complaint tends to outlive the airflow problem you were solving. The velocity check flags this as you size. Treat these as general guidance and confirm against Manual D, the manufacturer's blower table, and local code.

Rectangular duct is often the only option in a tight chase or above a hard ceiling. To compare the two, the calculator converts the rectangular size to an equivalent round diameter — the round duct that would have the same friction loss:

De = 1.3 × (a · b)^0.625 / (a + b)^0.25

Give it the height you actually have room for and it solves for the width that hits the equivalent diameter. Round duct is always more efficient for the same cross-section — go rectangular only when the space makes you.