Airflow Choke Point Calculator

This calculator helps identify where a duct system may be restricting airflow and causing static pressure problems.

Start by entering the furnace size, furnace type, and A/C tonnage. The calculator compares the airflow required by the furnace and the A/C, then uses the higher CFM number as the target airflow for the duct system.

Next, enter the existing ductwork sizes:

Supply plenum
Supply trunklines
Supply runs
Return trunklines
Return drop
Return grilles
Filters

The calculator will total the available CFM and show which sections are undersized. When a section is too small, it will recommend what to change it to or what to add. Once the ductwork entered is large enough for the required airflow, the warning will clear.

For return trunklines, choose whether the return trunk goes in one direction or both directions from the drop. If the trunk runs both ways, the calculator counts both sides of the trunkline toward the total return capacity.

Use this tool as a field guide to help find airflow choke points, improve delivered CFM, and reduce static pressure issues. Final sizing should always be verified with actual static pressure readings, airflow measurements, equipment data, duct fittings, equivalent length, and local code requirements.

Disclaimer

This calculator is provided as a general field guide only. It is not a substitute for a full HVAC design, Manual D calculation, manufacturer specifications, local code requirements, or professional judgment.

Actual airflow and static pressure can be affected by duct length, fittings, transitions, elbows, dampers, flex duct installation, filter type, grille design, equipment setup, blower speed, coil restriction, and installation conditions.

All recommendations should be verified with proper testing, including total external static pressure, delivered airflow, temperature rise, equipment data, and applicable local codes. RISE Coaching, the calculator provider, and any related parties are not responsible for system performance, code compliance, equipment damage, comfort issues, or installation decisions made from use of this tool.