đ§ Duct Size Calculator
Size your HVAC ductwork correctly. Calculate round duct diameter, rectangular equivalents, and static pressure based on CFM and duct specs.
How to Use This Calculator
Enter the required CFM (airflow) for the duct run â refer to the CFM reference chart below if you're unsure. Input the total duct length and number of bends in the run. Select your duct material type. The calculator determines the optimal duct size to deliver proper airflow without excessive noise or pressure loss.
Understanding Your Results
The round duct diameter is the standard recommendation. If round duct doesn't fit your space, use the rectangular equivalent dimensions. Static pressure loss indicates how much resistance the duct run creates â your blower must overcome this pressure. Keep total system static pressure under 0.5 inches of water column for most residential systems.
Frequently Asked Questions
What size duct do I need?
Duct size depends on the CFM (airflow) required and acceptable noise/velocity levels. Higher CFM needs larger ducts. Our calculator accounts for duct length, bends, and material type.
What is the difference between flex and rigid duct?
Rigid (sheet metal) ducts have lower friction and allow better airflow. Flex duct is easier to install but creates more resistance. Our calculator adjusts recommendations based on duct material.
How do I calculate duct static pressure?
Static pressure depends on duct size, length, material, velocity, and fittings (bends, transitions). Our calculator estimates total static pressure to help ensure your system performs properly.
Can I convert round duct to rectangular?
Yes! Any round duct has a rectangular equivalent that provides the same airflow. Our calculator provides both dimensions so you can choose what fits your space.