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How many CFM can a 6 inch duct carry?

About 177 CFM — at the top of the quiet residential range (900 fpm).

A 6 in round duct has 0.196 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 295 CFM, but that is loud in a home. There is no single "max CFM" for a duct — only the airflow at which it gets too noisy or too restrictive, which is what these numbers describe.

Engineering estimate. Clean galvanized round duct, standard air, straight duct only. Fittings and flex change these numbers substantially — verify against a full Manual D before fabricating. See disclaimer.

6 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 6 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
45 CFM 229 fpm 0.020 in. w.c. Within range
90 CFM 458 fpm 0.068 in. w.c. Within range
135 CFM 688 fpm 0.141 in. w.c. Within range
175 CFM 891 fpm 0.227 in. w.c. Within range
220 CFM 1,120 fpm 0.345 in. w.c. Runs fast
265 CFM 1,350 fpm 0.488 in. w.c. Runs fast

Velocity V = 183.3 × CFM / D². Friction from Darcy-Weisbach with the Haaland factor, clean galvanized (ε = 0.0003 ft). Residential systems are usually designed near 0.1 in. w.c. per 100 ft.

Rectangular and oval equivalents of 6 inch round

These carry the same air at the same friction loss — the equal-friction equivalents, not equal area. A rectangle always needs more cross-section than the round duct it replaces.

Rectangular and flat-oval ducts equivalent to 6 inch round.
Shape Size Aspect ratio
Round 6 in 1.0
Flat oval 8 × 7 in 1.1

Huebscher (rectangular) and Heyt & Diaz (flat oval). Keep aspect ratio under about 4:1 — past that, friction rises faster than the equivalence relation predicts.

What 6 inch duct is used for

The 6 inch mistake worth knowing

6 in is the default residential branch and the most over-assumed size in the trade. It is comfortable to about 100 CFM in the quiet band — well short of the 150 CFM many installers push through it.