About this calculator This calculator determines the round duct diameter needed for a given airflow and target velocity, or the velocity for a given duct size. Use it during duct layout to select duct diameters that keep velocities within design limits. Proper duct sizing balances noise levels, pressure drop, and material cost for the duct system.
Use when You need to size a round duct for a target friction rate.
Formula Iterative sizing to match target friction rate (in. W.C./100 ft)
Variables
CFM Airflow in cubic feet per minuteFriction Rate Target friction rate in in. W.C. per 100 ft (default 0.1)
Airflow (CFM)
Volume of air flow in cubic feet per minute
Target Friction (in.w.g./100ft)
Desired friction rate per 100 ft of duct
Diameter (in) —
Velocity (FPM) —
Actual Friction (in.w.g./100ft) —

How is round duct size calculated?

Round duct diameter is calculated from the required airflow and a target velocity using the relationship between flow, area, and velocity. The formula is: Diameter = √(4 × CFM ÷ (π × Velocity)), where CFM is the airflow in cubic feet per minute and Velocity is the target air speed in feet per minute (FPM). This derives from the basic flow equation Q = V × A, where A is the circular cross-sectional area (π × D² ÷ 4). Round ducts are preferred for many HVAC applications because they have the lowest surface-area-to-volume ratio of any duct shape, resulting in less friction loss per unit of airflow. Standard round duct diameters are manufactured in increments (typically 1-inch steps for small sizes and 2-inch steps for larger sizes), so the calculated diameter is rounded up to the next available standard size.

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Duct Velocity CFM