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How many CFM can an 8 inch duct carry?

About 314 CFM — at the top of the quiet residential range (900 fpm).

An 8 in round duct has 0.349 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 524 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.

8 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 8 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
80 CFM 229 fpm 0.014 in. w.c. Within range
155 CFM 444 fpm 0.045 in. w.c. Within range
235 CFM 673 fpm 0.095 in. w.c. Within range
315 CFM 902 fpm 0.162 in. w.c. Runs fast
395 CFM 1,132 fpm 0.246 in. w.c. Runs fast
470 CFM 1,346 fpm 0.340 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 8 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 8 inch round.
Shape Size Aspect ratio
Round 8 in 1.0
Rectangular × in NaN
Flat oval 10 × 6 in 1.7

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 8 inch duct is used for

The 8 inch mistake worth knowing

8 in is where flex duct starts costing you badly. The published CFM figures assume smooth galvanized pipe; flex with even slight compression can cut real capacity by a quarter or more at this diameter.