Duct Size Calculator
Round Duct from CFM
Calculate round duct diameter from CFM and target velocity
Rectangular Duct from CFM
Calculate rectangular duct dimensions from CFM
Formula
D = sqrt(4 x CFM x 144 / (pi x Velocity)) | Area = CFM / Velocity
Frequently Asked Questions
How do I determine the right duct size?
Duct size depends on required airflow (CFM) and maximum acceptable velocity. Residential ducts typically use 600-900 FPM for main trunks and 400-600 FPM for branches to keep noise acceptable.
What velocity should I use for duct sizing?
For residential systems, use 600-900 FPM for trunk lines and 400-600 FPM for branch runs. Commercial systems may use 1000-2000 FPM depending on noise requirements.
Is round or rectangular duct better?
Round duct has less friction loss per CFM and uses less material. Rectangular duct is useful where ceiling height is limited.
You may also need
~
Airflow CFM Calculator
Free airflow CFM calculator. Calculate cubic feet per minute for HVAC systems based on room size, duct velocity, or air changes per hour.
Everyday~
Static Pressure Drop Calculator
Free static pressure drop calculator. Estimate friction loss in ductwork based on duct size, length, and airflow.
Everyday~
Air Handler Sizing Calculator
Free air handler sizing calculator. Determine the right air handler capacity based on system tonnage, airflow requirements, and ductwork.
Everyday