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How many CFM can a 7 inch duct carry?

About 241 CFM — at the top of the quiet residential range (900 fpm).

A 7 in round duct has 0.267 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 401 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.

7 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 7 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
60 CFM 225 fpm 0.016 in. w.c. Within range
120 CFM 449 fpm 0.054 in. w.c. Within range
180 CFM 674 fpm 0.112 in. w.c. Within range
240 CFM 898 fpm 0.190 in. w.c. Within range
300 CFM 1,123 fpm 0.286 in. w.c. Runs fast
360 CFM 1,347 fpm 0.402 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 7 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 7 inch round.
Shape Size Aspect ratio
Round 7 in 1.0
Rectangular × in NaN
Flat oval 8 × 6 in 1.3

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 7 inch duct is used for

The 7 inch mistake worth knowing

7 in is stocked far less than 6 or 8 and is often skipped for that reason alone. That is a real cost: jumping 6 → 8 in overshoots by 78% in area where 7 in would have been the right answer.