Duct Sizing Chart CFM
Quick answer: this field chart estimates airflow for specific duct sizes under stated assumptions. At the chart’s listed rates, 8-inch round metal duct is shown at 180 CFM, 10-inch at 325 CFM, and 12-inch at 525 CFM. The flex-duct values differ. Use the chart to compare examples, not to size a complete HVAC system.
Round Duct Airflow Estimates
The source chart lists 0.06 in. w.c. per 100 ft for round metal pipe and 0.05 in. w.c. per 100 ft for flex duct. Its rectangular sheet-metal examples use 0.07 in. w.c. per 100 ft. Airflow is not determined by diameter alone; friction rate and installation conditions matter.
| 5 in | 50 CFM | 50 CFM | Small branch |
|---|---|---|---|
| 6 in | 85 CFM | 75 CFM | Small room branch |
| 7 in | 125 CFM | 110 CFM | Medium branch |
| 8 in | 180 CFM | 160 CFM | Common branch |
| 9 in | 240 CFM | 225 CFM | Large branch |
| 10 in | 325 CFM | 300 CFM | Small trunk |
| 12 in | 525 CFM | 480 CFM | Trunk line |
| 14 in | 750 CFM | 700 CFM | Trunk line |
| 16 in | 1,200 CFM | 1,000 CFM | Main trunk |
| 18 in | 1,500 CFM | 1,300 CFM | Main trunk |
| 20 in | 2,000 CFM | 1,700 CFM | Main trunk |
11 rows
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Rectangular Duct Examples
These are specific airflow-and-size pairs from the same field chart. They are selected examples, not interchangeable round-duct equivalents.
| 120 CFM | 10 x 4 in |
|---|---|
| 215 CFM | 10 x 6 in |
| 310 CFM | 8 x 10 in |
| 490 CFM | 14 x 8 in |
| 750 CFM | 20 x 8 in |
| 1,200 CFM | 30 x 8 in |
6 rows
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What Changes Duct Airflow
- Friction rate depends on the system pressure budget and total effective duct length.
- Elbows, transitions, takeoffs, dampers, filters, and registers add resistance.
- Flex duct capacity depends on its length, support, stretch, compression, and fittings.
- Air velocity and duct noise also matter when selecting a size.
For a residential system, use the equipment data, system layout, duct material, and fitting losses in an ACCA Manual D design. A field chart cannot confirm total airflow or blower performance; those should be checked for the installed system.
Sources
The airflow examples and listed friction rates are from HVACDirect’s Field Duct Sizing Chart. ASHRAE’s duct-design chapter explains how duct friction and flexible-duct installation affect resistance. For complete residential sizing, see ACCA Manual D.
