Understanding Flex Duct Size
Flexible duct carries conditioned air through bends, framing gaps, and short branch runs. Correct sizing keeps air moving at a controlled speed. It also protects fan performance. A duct that is too small raises velocity. Noise rises, static pressure increases, and rooms may receive less airflow. A duct that is too large can waste space and may reduce throw from registers.
Why Airflow And Velocity Matter
This calculator starts with airflow in cubic feet per minute. It then divides that airflow by the target velocity. The result is required area. A round diameter is found from that area. The tool also checks an existing duct size, so you can compare its actual velocity. This is useful when replacing a branch, checking a design, or reviewing a field change.
Flex Duct Physics
Air does not move through flexible duct without resistance. Bends, poor stretching, long runs, and fittings all add pressure loss. Fully stretched duct is usually smoother than compressed duct. The calculator accounts for this by increasing equivalent length when stretch quality is low. It then applies the selected friction rate to estimate total loss. This is an estimate, not a final certified design.
Practical Design Notes
Use the result as a planning guide. Keep flex runs short when possible. Avoid tight bends. Support the duct so it stays round. Seal joints carefully. Match grille, boot, and trunk sizes with the same airflow goal. A quiet branch often needs lower velocity than a main supply run. Return ducts may also need conservative sizing because noise can travel back to the equipment.
Field Review Tips
Check the route before ordering material. Measure clearances, ceiling depth, hanger spacing, and connection collars. Small field details can change the final pressure result quickly.
Using The Output
The suggested diameter rounds up from the calculated value. Rounding up helps keep velocity below the target. The pressure loss result helps compare several layouts. The velocity pressure value shows how much dynamic air energy exists in the stream. CSV export is useful for spreadsheets. PDF export is useful for job notes and customer files. Always confirm important projects with local codes, equipment data, and a qualified HVAC professional before installation on every job.