Duct Size Calculator

A 11 in round duct carries 400 CFM at the common 0.08 in. wc per 100 ft friction rate (galvanized, standard air). That is roughly one ton of cooling airflow at 400 CFM per ton. At a looser 0.10 in. wc/100 ft you can drop a size; a rectangular duct with the same equivalent diameter carries the same airflow at the same friction.

An 14 in round duct carries 800 CFM at 0.08 in. wc per 100 ft. Returns and trunks are often run a size larger to keep velocity and noise down. Use the calculator to see the exact velocity and to read off rectangular equivalents for a tight joist bay.

Duct size comes from the equal-friction method: pick a friction rate (0.08 in. wc per 100 ft is the residential default), then find the smallest standard duct whose friction loss at your CFM stays at or under that rate. Friction loss is (12 f L / D) x density x (velocity/1097) squared, with the friction factor f from the Colebrook-White equation at galvanized roughness 0.0003 ft. This calculator solves it for you and gives the round size, the velocity, and the rectangular equivalent.

0.08 in. wc per 100 ft is the standard residential design target, and ACCA Manual D Appendix 3 tables are built on it. The usable range is about 0.06 to 0.10 in. wc per 100 ft: lower means larger, quieter ducts, higher means smaller ducts and more noise. Your actual friction rate is available static pressure times 100 divided by total effective length.

A 6 in round duct carries roughly 90 CFM at 0.08 in. wc per 100 ft before it needs to step up to 7 in. Push more air through it and the velocity and friction climb past the residential target, which is where duct noise starts. The chart on this page lists each round size against the airflow it carries.

Round duct moves air with less friction and material for the same airflow, but rectangular fits flat in joist bays and above ceilings. To convert, match the equivalent diameter: De = 1.30 times (a times b) to the 0.625 power, divided by (a plus b) to the 0.25 power (ASHRAE Fundamentals 21.3.1). The calculator prints the rectangular sizes whose equivalent diameter matches the round size it recommends.