Duct Size Calculator
How It Works
Proper duct sizing ensures efficient HVAC performance and comfortable airflow. Undersized ducts create excessive noise and pressure drop, while oversized ducts waste material and can cause low air velocity that leads to temperature stratification.
The calculation is based on the fundamental equation: Area = CFM / Velocity. For residential systems, main trunk ducts typically run at 700-900 FPM, while branch ducts to individual rooms run at 400-600 FPM. Commercial systems may use higher velocities.
Round ducts are the most efficient shape for airflow as they minimize friction. Rectangular ducts are used when ceiling space is limited. When converting between shapes, maintain the same cross-sectional area for equivalent performance.
Frequently Asked Questions
What CFM do I need per room?
A general rule is 1 CFM per square foot of living space. A 200 sq ft bedroom needs about 200 CFM, while a 300 sq ft living room needs 300 CFM. Kitchens and rooms with high heat gain may need more.
What is the maximum duct velocity for residential systems?
For residential comfort, main ducts should not exceed 700-900 FPM and branch ducts 400-600 FPM. Higher velocities create noise — keep velocities under 600 FPM for bedrooms and quiet spaces.
Can I reduce duct size to save space?
Reducing duct size increases air velocity and noise, and creates more pressure drop which makes your HVAC system work harder. Always size ducts to meet airflow requirements within acceptable velocity limits.
Round vs rectangular ducts — which is better?
Round ducts have lower friction losses, are easier to seal, and use less material for the same airflow. Rectangular ducts fit in tight spaces with limited height. Use round when possible for best efficiency.