Duct sizing is the process of calculating appropriate ductwork dimensions to deliver required airflow while maintaining acceptable pressure drops and velocities. Properly sized ducts deliver design CFM specifications throughout all zones while minimizing energy losses and noise generation. Undersized or oversized ducts compromise system efficiency, comfort, and equipment longevity.
Sizing Methodology
Engineers calculate duct size using system CFM requirements and maximum acceptable velocity limits. Residential main ducts typically operate at 800-1,200 CFM with velocities of 900-1,100 feet per minute. Branch ducts carry 200-500 CFM at velocities of 600-900 feet per minute. Professional sizing requires detailed building layouts, accurate load calculations, and friction loss calculations. Ductwork area directly correlates with installation costs and space requirements.
Design Consequences
Undersized ducts increase air velocity, creating excessive friction losses and noise generation. A 6-inch round duct carrying 500 CFM experiences unacceptable noise and pressure drop compared to a properly sized 8-inch duct. Oversized ducts waste installation costs and space without performance benefits. Proper sizing balances airflow delivery, energy efficiency, and noise control. Most residential systems use ductwork sized to 1.0 to 1.2 inches of water column total pressure drop.