Flex Duct Sizing Chart: Airflow, Diameter & Velocity
Compare airflow and velocity for round flex duct diameters, then check product pressure-loss data, compression, bends and the available static-pressure budget.
Compare velocities before checking pressure loss
Each cell is airflow divided by the full round internal area. Diameter means the clear internal diameter, not the outside of the insulation. Values are geometry calculations, not recommended airflow capacities or a guarantee that the fan can deliver them.
| Inside diameter (in) | 100 CFM → FPM | 200 CFM → FPM | 400 CFM → FPM | 600 CFM → FPM |
|---|---|---|---|---|
| 6 | 509 | 1019 | 2037 | 3056 |
| 7 | 374 | 748 | 1497 | 2245 |
| 8 | 286 | 573 | 1146 | 1719 |
| 9 | 226 | 453 | 905 | 1358 |
| 10 | 183 | 367 | 733 | 1100 |
| 12 | 127 | 255 | 509 | 764 |
| 14 | 94 | 187 | 374 | 561 |
| 16 | 72 | 143 | 286 | 430 |
Worked example: 200 CFM through an 8-inch inner core
A = π × (8/12)² / 4 ≈ 0.3491 ft². V = 200 / 0.3491 ≈ 573 FPM. At the same flow a 10-inch core gives about 367 FPM. This comparison says nothing about the installed pressure drop until product, length, compression and bends are known.
Use the actual flex product loss data
- Record the required room airflow and available pressure for the run.
- Read the selected manufacturer’s pressure-loss data at that airflow and diameter, with its stated test conditions.
- Account for installed length, compression, bends, fittings and terminals without double-counting losses already included in the data.
- Compare the complete return/supply path with the blower budget and verify delivered airflow after installation.
The Air Duct Council installation standard, sixth edition, section 4.6 explains that longitudinal or circumferential compression increases pressure loss. Follow the exact product’s installation instructions. A smooth-metal Darcy estimate and a blanket “add one inch” rule do not establish installed flex performance.
No friction value is supplied here because diameter and airflow alone cannot determine real flex pressure loss.
How to use this flex duct velocity comparison
- Choose the design airflow column.
- Compare candidate internal diameters and their calculated velocities.
- Finish selection with manufacturer loss data and the complete pressure budget.
Worked example: This table is not a flex friction chart or a final Manual D selection.
Record the result with the project address, equipment model, code edition and assumptions. That audit trail is more useful than copying a single number without its conditions.
Continue with the right tool
Use boundary: No product-specific roughness, compression correction, fitting loss, pressure or acoustic rating is inferred.
Cite or reuse this table
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HVAC Calculator Pro. (2026-10-08). Flex Duct Sizing Chart: Airflow, Diameter & Velocity. Retrieved from https://hvaccalcpro.com/charts/flex-duct-sizinghttps://hvaccalcpro.com/charts/flex-duct-sizing