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How many CFM can a 9 inch duct carry?

About 398 CFM — at the top of the quiet residential range (900 fpm).

A 9 in round duct has 0.442 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 663 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.

9 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 9 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
100 CFM 226 fpm 0.012 in. w.c. Within range
200 CFM 453 fpm 0.040 in. w.c. Within range
300 CFM 679 fpm 0.083 in. w.c. Within range
400 CFM 905 fpm 0.141 in. w.c. Runs fast
495 CFM 1,120 fpm 0.209 in. w.c. Runs fast
595 CFM 1,347 fpm 0.295 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 9 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 9 inch round.
Shape Size Aspect ratio
Round 9 in 1.0
Rectangular × in NaN
Rectangular × in NaN
Flat oval 14 × 6 in 2.3

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 9 inch duct is used for

The 9 inch mistake worth knowing

9 in sits in the awkward gap where many size charts jump straight from 8 to 10. It exists and is stocked, and using it avoids the 23% area overshoot of going to 10 in.