Ductwork Sizing Calculator
Size round duct from airflow and target air velocity.
How this calculator works
area = CFM ÷ velocity · diameter = √(4 × area ÷ π)
Round duct at the same area carries more air than rectangular because it has less surface friction per unit of flow. Round up to the next standard size: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16 inches.
Worked example
With airflow 400 , target velocity 700 , this calculator returns 10.24 in.
Frequently asked questions
What size duct do I need for 400 CFM?
About a 10 inch round duct at 700 fpm, so use the next standard size up. Pushing the same 400 CFM through an 8 inch duct raises velocity to 1,150 fpm and makes it audible.
What velocity should residential ductwork run at?
Six hundred to 900 fpm in branches and 700 to 1,200 in trunks. Above roughly 900 fpm at a register, air noise becomes noticeable in a quiet room.
Is round or rectangular duct better?
Round, on every measure except fitting into a joist bay. It has less friction per unit of flow, less surface area for heat loss and no corners for turbulence. Use rectangular only where space forces it.
Does flexible duct need to be larger?
Yes, effectively. Flex duct has roughly three times the friction of smooth metal even when pulled tight, and far more when it sags. Size up one increment and keep runs short and straight.