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How many CFM can an 18 inch duct carry?

About 1,590 CFM — at the top of the quiet residential range (900 fpm).

An 18 in round duct has 1.767 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 2,651 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.

18 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 18 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
400 CFM 226 fpm 0.005 in. w.c. Within range
795 CFM 450 fpm 0.017 in. w.c. Within range
1,195 CFM 676 fpm 0.035 in. w.c. Within range
1,590 CFM 900 fpm 0.060 in. w.c. Within range
1,990 CFM 1,126 fpm 0.091 in. w.c. Runs fast
2,385 CFM 1,350 fpm 0.127 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 18 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 18 inch round.
Shape Size Aspect ratio
Round 18 in 1.0
Rectangular × in NaN
Rectangular × in NaN
Rectangular × in NaN
Flat oval 26 × 12 in 2.2

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 18 inch duct is used for

The 18 inch mistake worth knowing

18 in duct moves serious air, and at these volumes the velocity band that matters is the commercial one, not the residential. Applying the 900 fpm residential limit here typically oversizes the duct and wastes plenum space for no acoustic benefit.