Duct Sizing Calculator (Equal Friction Method)
Calculate duct sizing using the equal friction method. Designed for HVAC engineers and professionals.
Airflow & Friction
Provide the target airflow in CFM and the allowable friction loss per 100 feet of duct.
Typical systems range from 500 to 2500 CFM per main branch.
Common design targets are 0.08 to 0.12 in. w.g.
Assumes a constant velocity of 400 feet per minute under the equal friction method.
How to Use This Calculator
Enter the design airflow you need to move through the duct and the allowable friction loss per 100 feet. Press Calculate or wait for the automatic update to see how large the duct must be to keep the flow close to the equal friction target.
The reset button restores the default sample scenario so you can compare your system quickly.
Methodology
This tool assumes an equal friction design: the ductwork is sized so that the pressure drop per unit length stays roughly constant. The calculator targets 400 feet per minute, a common balance point for residential and light commercial runs, and back-computes the required diameter from the airflow.
Results are estimates for preliminary design. Use them alongside manufacturer charts and confirm with detailed HVAC sizing procedures.
Full original guide (expanded)
Data Source and Methodology
All calculations are based on the ASHRAE Fundamentals Handbook.
Glossary of Variables
- Airflow (CFM): The volume of air flowing through the duct per minute.
- Friction Loss: The pressure loss due to friction in the duct.
How It Works: A Step-by-Step Example
For an airflow of 1000 CFM and a friction loss of 0.1, the calculated duct diameter is 10 inches.
Frequently Asked Questions (FAQ)
What is the equal friction method?
The equal friction method is a technique used in HVAC design to size ducts based on a constant pressure loss per unit length.