Flexible duct is quick to run, but it does not move as much air as smooth metal pipe of the same diameter, and the gap grows fast if the duct is compressed or left sagging. This chart shows the airflow each round size carries at a 0.10 in. w.g. per 100 ft friction rate, the common residential design target.
| Round size | Flex CFM | Metal CFM |
|---|---|---|
| 4" | 25 | 35 |
| 5" | 45 | 65 |
| 6" | 75 | 110 |
| 7" | 115 | 165 |
| 8" | 165 | 240 |
| 9" | 230 | 325 |
| 10" | 305 | 435 |
| 12" | 495 | 710 |
| 14" | 755 | 1,070 |
| 16" | 1,080 | 1,525 |
Why flex carries less
A flex duct's corrugated inner liner is rougher than sheet metal, so even pulled tight it loses more pressure. The bigger issue is compression: when flex is not stretched taut and supported, the liner bunches and the effective friction climbs.
- Pulled tight (~4% compression): friction ≈ 1.9× metal.
- Left ~15% compressed: friction ≈ 3× metal, roughly half the airflow.
In practice that means a run of flex usually needs to be one to two sizes larger than the metal it replaces to move the same air. Compare the two columns above: 8" flex carries about what 7" metal does.
How to use the chart
Find the airflow the run must carry (from a room-by-room load, not a rule of thumb), then pick the smallest flex size whose CFM meets it at your design friction rate. If your friction rate isn't 0.10 (say you have a high-static furnace or a long duct system), size it for your actual rate rather than reading this chart.