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Ubify Media & Design Operations
UI/UX & DIGITAL MEDIA SPECIALISTS
About This Tool
The HVAC Duct CFM Sizer calculates round spiral pipe diameter and equivalent rectangular sheet metal dimensions using available static pressure friction rates and design airflow volumes.
How the Duct Sizer Works: The calculator applies the ACCA Manual D duct sizing formula to evaluate continuous airflow volume against friction loss per 100 equivalent feet of ductwork. Entering design CFM and available friction rate computes the minimum round duct diameter, total cross-sectional area, and resulting air velocity in feet per minute (FPM).
Equivalent Rectangular Dimensions: Because floor joists and ceiling plenums often lack vertical clearance for large round pipes, the tool calculates equivalent rectangular dimensions for standard 8-inch, 10-inch, and 12-inch depth profiles with equal cross-sectional flow area.
Worked Example: For a residential supply trunk carrying 400 CFM at a target friction rate of 0.08 inches water column per 100 feet, the calculator determines a 10-inch round duct diameter with an air velocity of approximately 733 FPM. For tight horizontal clearances, equivalent rectangular profiles include an 8" × 10" or 10" × 8" rectangular trunk.
Execution & Privacy: All calculations execute client-side using JavaScript in your local browser session. No room dimensions, duct configurations, or HVAC layouts are transmitted to remote servers.
How to Use
- 1
Enter the required room or zone airflow in cubic feet per minute (CFM).
- 2
Enter the target friction rate in inches of water gauge per 100 feet (typically 0.08 in. wg for residential or 0.10 in. wg for commercial runs).
- 3
Review the calculated round duct diameter, internal cross-sectional area, and resulting air velocity in feet per minute (FPM).
- 4
Inspect equivalent rectangular duct profiles (8-inch, 10-inch, or 12-inch depth options) for tight ceiling cavity installations.
Frequently Asked Questions
What is a typical friction rate for residential HVAC duct design?
Standard residential duct systems are typically designed using a friction rate between 0.06 and 0.08 inches of water column (in. wg) per 100 equivalent feet. Commercial supply systems often use 0.10 in. wg per 100 feet.
How much airflow CFM is required per ton of air conditioning?
Standard residential split-system cooling equipment requires between 350 and 450 CFM per nominal ton (12,000 BTU/hr) of cooling capacity, with 400 CFM per ton being the standard design rule of thumb.
Why is air velocity in feet per minute (FPM) important in branch ducts?
Air moving faster than 600 to 700 FPM through residential branch ducts and supply registers causes noticeable air rushing noise in bedrooms and living areas.
Does this duct calculator send project designs to a remote server?
No. The sizing equations run locally in your web browser with zero server data transfer.