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How many CFM can a 16 inch duct carry?

About 1,257 CFM — at the top of the quiet residential range (900 fpm).

A 16 in round duct has 1.396 ft² of free area. Pushed to the commercial velocity cap (1500 fpm) it will pass roughly 2,094 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.

16 inch duct: airflow, velocity and friction rate

Velocity and friction rate for a 16 inch round duct at several airflows.
Airflow Velocity Friction / 100 ft Verdict
315 CFM 226 fpm 0.006 in. w.c. Within range
630 CFM 451 fpm 0.020 in. w.c. Within range
940 CFM 673 fpm 0.040 in. w.c. Within range
1,255 CFM 899 fpm 0.069 in. w.c. Within range
1,570 CFM 1,124 fpm 0.104 in. w.c. Runs fast
1,885 CFM 1,350 fpm 0.147 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 16 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 16 inch round.
Shape Size Aspect ratio
Round 16 in 1.0
Rectangular × in NaN
Rectangular × in NaN
Rectangular × in NaN
Flat oval 24 × 10 in 2.4

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 16 inch duct is used for

The 16 inch mistake worth knowing

At 16 in the duct itself is rarely the restriction — fittings are. A single sharp elbow at this diameter can add more effective length than 20 ft of straight pipe, which is why total effective length, not run length, drives the design.