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.