Duct Sizing Calculator - Round Duct and Rectangular Equivalents 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 for a target friction-rate screen or velocity limit for a maximum-velocity calculator. The app does not run static regain or a full system pressure model.

3. Enter Design Constraint

   For equal friction, enter the target friction rate in in. w.g. per 100 ft. For velocity mode, enter the maximum velocity in FPM. These inputs must come from a real design basis.

4. Review Duct Sizes

   The calculator outputs a calculated round diameter, local standard round size, velocity, friction estimate, and equivalent rectangular candidates. Verify all sizes against the full duct design.

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

Two Screening Modes

Equal-friction mode sizes from CFM and target friction rate. Velocity-limit mode sizes from CFM and maximum velocity. Static regain is not implemented.

Round and Rectangular Output

Returns a calculated round diameter, local standard round size, and rectangular equivalent candidates screened by equivalent diameter and aspect ratio.

Velocity and Noise Warning

Flags high selected-size velocities as a planning warning. Final noise and throw checks require diffuser, grille, room, and system design review.

Friction Estimate

Shows friction estimate per 100 ft at the selected local round size. It is not a total pressure-drop or fitting-loss calculation.

PDF Export

Export the screening result and warnings as a PDF for internal planning notes. Do not use it alone as a permit design or construction document.

Assumptions

• Airflow is entered by the user for one duct segment.
• Equal-friction mode uses a simplified local round-duct formula with friction rate in in. w.g. per 100 ft.
• Velocity-limit mode sizes a round duct from CFM and maximum velocity.
• Rectangular equivalents are equivalent-diameter candidates and not construction approval.
• Velocity references are planning context only and require project validation.

Limitations

• Does not perform static regain calculations.
• Does not calculate total external static pressure, fan operating point, or equipment selection.
• Does not add fitting equivalent lengths or loss coefficients.
• Does not apply flex duct correction for compression, sag, bend radius, support, or installed length.
• Does not model leakage, balancing, diffuser/grille selection, fire/smoke dampers, insulation, duct construction, or code/AHJ acceptance.

References

• ACCA Manual D residential duct design source pointer.
• ASHRAE Handbook duct design chapter source pointer.
• SMACNA HVAC duct construction standards source pointer.
• DOE/PNNL Building America Manual D source pointer.

Frequently Asked Questions