Duct Size Calculator

Size round and rectangular ducts from airflow. Enter CFM and a target velocity to get the round duct diameter and an equivalent rectangular size for a chosen height.

Airflow

Sizing Method

Airflow (CFM)

Velocity (FPM)

700–900 supply trunk

Rect. Duct Height (in)

For the equal-friction width

Duct Size

Round Duct9 in Ø

Duct Area64 in²

Rectangular (equal friction)8.5 × 8 in

Equivalent Diameter9 in Ø

Rect. Velocity847.06 fpm

Round: A = CFM ÷ velocity. Rectangular by ASHRAE equal-friction De = 1.30·(a·b)^0.625 ÷ (a+b)^0.25 (not equal area). Confirm vs Manual D.

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