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How many CFM can a 4 inch duct carry?

About 79 CFM — at the top of the quiet residential range (900 fpm).

A 4 in round duct has 0.087 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 131 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.

4 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 4 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
20 CFM 229 fpm 0.033 in. w.c. Within range
40 CFM 458 fpm 0.113 in. w.c. Within range
60 CFM 688 fpm 0.235 in. w.c. Within range
80 CFM 917 fpm 0.396 in. w.c. Runs fast
100 CFM 1,146 fpm 0.597 in. w.c. Runs fast
120 CFM 1,375 fpm 0.836 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 4 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 4 inch round.
Shape Size Aspect ratio
Round 4 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 4 inch duct is used for

The 4 inch mistake worth knowing

4 in is the smallest duct sold in volume and it is almost always undersized for what people attach to it. Bath fans rated 80–110 CFM are routinely put on 4 in flex, which pushes velocity past the residential band and turns the fan noisy while moving far less air than the label claims.