Duct Size Calculator
What Is the Duct Size?
Duct sizing refers to the calculation of the appropriate diameter for your system’s ductwork based on how much air (measured in CFM) needs to be transported and at what velocity (measured in feet per minute, or FPM). The goal is to select a duct size that allows for proper dust conveyance without excessive friction loss or particle dropout.
Well-designed hoods should minimize turbulence, maintain smooth airflow, and include air locks to
prevent blowback into the system.
How to Use This Calculator
⦿ Enter the System Airflow (CFM): Input the total volume of air moving through the specific duct branch or main trunk.
⦿ Enter the Target Air Velocity (FPM): Input the desired speed of the air (often 3,500 to 4,500 FPM for dust collection, depending on the material).
- ⦿ Calculate: Click the calculate button to determine the exact duct diameter needed to maintain that velocity at your given airflow. You can then use the buttons below the calculator to email yourself a PDF summary of the results or request a free system sizing consultation from our engineering team.
Understanding the Formula
Duct Diameter = 2 × √((CFM ÷ Velocity × 144) ÷ π)
Proper duct sizing is essential in dust collection to maintain the correct “conveying velocity.” If a duct is too large, the air slows down, and dust will settle out of the airstream and clog the pipe. If a duct is too small, the air moves too fast, causing severe abrasion and wasting fan energy through high static pressure.
This formula calculates the required cross-sectional area (by dividing CFM by Velocity), converts it to square inches, and then uses the geometry of a circle to solve for the required internal duct diameter in inches.
Importance of Duct Size In Baghouse Design
Poor duct sizing can lead to a number of costly issues. If the duct is too small, it can create excessive static pressure, overloading the fan and reducing system performance. If it’s too large, air velocity may drop below the minimum transport velocity, causing dust to settle and clog the ductwork.
Correct duct sizing helps maintain transport velocity above minimum values (typically 3,500–4,500 FPM, depending on material). It prevents dust accumulation and blockages as well as reducing energy use by avoiding oversized fans.
Note: While this formula provides a solid starting point for duct sizing, it should not be used as the only basis for design decisions. There are many other factors a dust collection expert considers, including duct length and layout, number of branches and elbows, material characteristics (abrasive, sticky, etc) and fan capacity and system static pressure.Â
We strongly recommend consulting with a dust collection expert, such as the team at Baghouse.com, before finalizing any ductwork layout. We can help tailor your system to your specific plant requirements and avoid costly design errors.