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How many CFM can a 5 inch duct carry?

About 123 CFM — at the top of the quiet residential range (900 fpm).

A 5 in round duct has 0.136 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 205 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.

5 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 5 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
30 CFM 220 fpm 0.023 in. w.c. Within range
60 CFM 440 fpm 0.079 in. w.c. Within range
90 CFM 660 fpm 0.165 in. w.c. Within range
125 CFM 917 fpm 0.300 in. w.c. Runs fast
155 CFM 1,137 fpm 0.445 in. w.c. Runs fast
185 CFM 1,357 fpm 0.618 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 5 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 5 inch round.
Shape Size Aspect ratio
Round 5 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 5 inch duct is used for

The 5 inch mistake worth knowing

5 in is the quiet upgrade from 4 in and costs almost nothing extra. Going from 4 to 5 in increases area by 56%, which drops velocity by more than a third at the same airflow — the cheapest noise fix in residential ductwork.