Ductwork Size Calculator

How It Works

Ductwork is the highway system of your HVAC — undersized ducts starve rooms of air, create noise, and force your system to work harder. The key measurement is CFM (cubic feet per minute), which is the volume of air flowing through the duct. Each room needs a specific CFM based on its BTU load. Duct sizing balances two competing goals: keeping air velocity low enough to avoid noise (under 900 feet per minute for branch ducts) while keeping duct diameter small enough to fit in walls and ceilings. Flex duct has higher friction than rigid metal, so it needs to be 1 to 2 inches larger for the same airflow.