Duct Sizing Calculator — Round, Rectangular & Flex Duct from CFM

How It Works

1. Determine Required CFM

   Calculate the airflow each duct run needs to deliver. For room branches, use the room-by-room CFM from a Manual D calculation or a CFM balancing tool. For trunk ducts, sum the CFM of all downstream branches. Typical residential rooms need 80-200 CFM depending on size, exposure, and heat load.

2. Select Sizing Method

   Choose equal friction (most common for residential and light commercial), velocity method (for industrial or noise-sensitive applications), or static regain (for long commercial trunk runs). Equal friction at 0.08 in. w.g. per 100 ft is the standard residential default. Higher friction rates allow smaller ducts but increase noise and blower energy.

3. Choose Duct Material

   Select galvanized sheet metal, flex duct, or fiberglass duct board. The material affects the friction factor and the absolute roughness used in sizing calculations. Flex duct is rougher and requires larger sizes for the same CFM.

4. Enter System Constraints

   Specify maximum velocity (to prevent noise), maximum friction rate, and available static pressure if sizing within a total system pressure budget. For residential, keep branch velocities under 600 fpm and trunk velocities under 900 fpm.

5. Review Duct Sizes

   The calculator outputs round duct diameter, velocity at that diameter, friction loss per 100 ft, and equivalent rectangular dimensions for common aspect ratios. If using flex, the corrected diameter accounts for the higher friction factor. Use these sizes for your duct layout.

Built For

• HVAC contractors sizing supply and return ductwork for new residential construction and renovation projects
• DIY installers adding or extending ductwork to a new room, addition, or finished basement
• Commercial HVAC engineers designing trunk-and-branch duct systems for office buildings and retail spaces
• HVAC technicians troubleshooting noise, poor airflow, or comfort complaints by verifying existing duct sizes against required CFM
• Home inspectors checking whether installed ductwork is adequately sized for the HVAC equipment capacity
• Energy auditors identifying undersized ductwork as a source of high blower energy and comfort problems

Features & Capabilities

Three Sizing Methods

Equal friction method sizes all ducts to maintain the same friction rate per 100 feet. Velocity method sizes to hit a target velocity. Static regain method reduces velocity in each section to recover pressure lost to friction, maintaining even pressure throughout the system.

Round and Rectangular Output

Returns the optimal round duct diameter and the equivalent rectangular dimensions for common aspect ratios (2:1, 3:1, 4:1). Rectangular equivalents maintain the same friction and airflow as the round size. Useful when ceiling space limits duct height.

Flex Duct Correction

Automatically applies a correction factor for flex duct installations. Flex duct has significantly higher friction than sheet metal due to the corrugated inner liner. The calculator upsizes the diameter to maintain the required CFM at the specified friction rate.

Velocity and Noise Check

Flags duct sizes where velocity exceeds recommended limits for the application. Residential branch ducts above 600 fpm and trunk ducts above 900 fpm generate perceptible noise. The calculator warns you before you build a noisy system.

Friction Loss Output

Shows friction loss per 100 feet of equivalent length at the selected duct size. Use this to check that your total duct system fits within the available external static pressure budget from the air handler or furnace.

PDF Export

Export your duct sizing results as a branded PDF for inclusion in HVAC proposals, permit applications, or construction documents.

Frequently Asked Questions