Round Duct Sizing

Round duct is sized by the equal-friction method: choose a friction loss rate per 100 ft of duct, then find the diameter at which the design airflow produces that loss. Friction loss in galvanized round duct at standard air follows Δh = 0.109136 × CFM^1.9 ÷ D^5.02 (in. w.c. per 100 ft, with D in inches), so solving for diameter gives D = (0.109136 × CFM^1.9 ÷ Δh)^(1/5.02). Because duct is manufactured in whole-inch sizes, the result is rounded up to the next inch, which puts the actual friction slightly below the target. Velocity then follows from the chosen size as CFM ÷ duct area. Smaller ducts cost less but run at higher velocity, which raises friction loss (roughly with the square of velocity), fan energy, and noise; the equal-friction target is how designers balance those. Round ducts are preferred where space allows because they have the lowest surface-area-to-volume ratio of any duct shape and so the least friction for a given airflow. For rectangular duct, convert width and height to an equivalent round diameter first.