Size round and rectangular ducts from airflow and target velocity.
Duct area follows directly from flow divided by velocity, and the round diameter comes from that area. Because pressure drop rises with the square of velocity, undersizing a duct is penalised twice. Velocity above about 7 m/s in residential ducts becomes audible, which is why quiet systems use larger ducts rather than more fan power.
Duct Size
Area = flow ÷ velocity; round diameter = √(4A ÷ π)
Area = flow ÷ velocity; round diameter = √(4A ÷ π) Duct area follows directly from flow divided by velocity, and the round diameter comes from that area. Because pressure drop rises with the square of velocity, undersizing a duct is penalised twice.
Velocity above about 7 m/s in residential ducts becomes audible, which is why quiet systems use larger ducts rather than more fan power.
This calculator takes 4 inputs: Airflow, Target velocity, Rectangular duct height, Duct run length. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.