Flexible Duct Sizing Chart
Flex duct is rougher than sheet metal, so it needs a larger size for the same air. At 0.1 in. w.g. per 100 ft,400 CFM needs 12 inch flex where sheet metal only needs 10 inch, and 1000 CFM needs 16 inch flex against 14 inch metal.Every number below assumes the flex is pulled fully tight. If it sags, none of it holds.
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Read this before you use the table
Flex duct has an absolute roughness around 10 times that of galvanized steel, because the inner liner rides over a wire helix instead of sitting flat. At the same diameter and airflow that works out to roughly1.4 times the friction. The chart already accounts for it, which is why the sizes run larger than a sheet metal chart.
What the chart cannot account for is slack. Fully extended flex is the published condition and it is the only condition these numbers describe. Flex left with a few extra feet coiled at the boot, or sagging between supports, has resistance well beyond anything in any table, and it does not show up on a walk through because it is above the ceiling. Sizing the pipe correctly and then installing it loose gets you a system that measures wrong and nobody can explain.
Flex duct size by CFM
Assumptions:
- Friction rate 0.1 in. w.g. per 100 ft, equal friction method
- Fully extended flex, absolute roughness 0.003 ft
- Standard air, 70°F at sea level
- Straight run only. Bends in flex cost far more than the same bend in hard pipe
| CFM | Round | Velocity fpm | Friction in. w.g./100 ft | Rectangular | Low profile |
|---|---|---|---|---|---|
| 25 | 4" | 286 | 0.065 | 4 x 4 | — |
| 50 | 5" | 367 | 0.077 | 6 x 4 | 8 x 4 |
| 75 | 6" | 382 | 0.065 | 6 x 6 | 12 x 4 |
| 100 | 7" | 374 | 0.051 | 8 x 8 | 16 x 4 |
| 125 | 7" | 468 | 0.078 | 8 x 8 | 16 x 4 |
| 150 | 8" | 430 | 0.056 | 8 x 8 | 16 x 4 |
| 175 | 8" | 501 | 0.075 | 8 x 8 | 16 x 4 |
| 200 | 8" | 573 | 0.097 | 8 x 8 | 16 x 4 |
| 250 | 9" | 566 | 0.081 | 10 x 10 | 18 x 6 |
| 300 | 10" | 550 | 0.067 | 10 x 10 | 24 x 6 |
| 350 | 10" | 642 | 0.091 | 10 x 10 | 24 x 6 |
| 400 | 12" | 509 | 0.046 | 12 x 12 | 24 x 6 |
| 450 | 12" | 573 | 0.058 | 12 x 12 | 24 x 6 |
| 500 | 12" | 637 | 0.071 | 12 x 12 | 24 x 6 |
| 600 | 14" | 561 | 0.045 | 14 x 14 | 30 x 8 |
| 700 | 14" | 655 | 0.061 | 14 x 14 | 30 x 8 |
| 800 | 14" | 748 | 0.079 | 14 x 14 | 30 x 8 |
| 900 | 14" | 842 | 0.100 | 14 x 14 | 30 x 8 |
| 1000 | 16" | 716 | 0.062 | 16 x 16 | 30 x 8 |
| 1200 | 16" | 859 | 0.088 | 16 x 16 | 30 x 8 |
| 1400 | 18" | 792 | 0.064 | 18 x 18 | 30 x 10 |
| 1600 | 18" | 905 | 0.084 | 18 x 18 | 30 x 10 |
| 1800 | 20" | 825 | 0.061 | 20 x 20 | 30 x 12 |
| 2000 | 20" | 917 | 0.075 | 20 x 20 | 30 x 12 |
| 2500 | 22" | 947 | 0.071 | 22 x 22 | 30 x 14 |
| 3000 | 24" | 955 | 0.065 | 22 x 22 | 30 x 18 |
| 3500 | 24" | 1114 | 0.087 | 22 x 22 | 30 x 18 |
| 4000 | 26" | 1085 | 0.075 | 24 x 24 | 30 x 20 |
Rectangular columns are shown for reference only. Flex duct is round; if you are looking at rectangular you are looking at sheet metal or duct board, and the sheet metal chart is the right one.
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Flex duct questions
What size flex duct do I need for 400 CFM?
A 12 inch flex duct at 0.1 in. w.g. per 100 ft, assuming it is pulled fully tight. The same 400 CFM through galvanized sheet metal only needs 10 inch. Flex is rougher, so it needs the larger size.
How much airflow does flex duct lose compared to sheet metal?
At the same diameter and the same airflow, flex duct has about 1.4 times the friction of galvanized steel, because its absolute roughness is roughly 10 times higher. That is why the sizes on this chart run one to two sizes larger than a sheet metal chart for the same CFM.
Does flex duct have to be pulled tight?
Yes, and it is the single thing most likely to make this chart wrong on your job. These numbers assume fully extended flex. Flex that sags between supports or is left compressed to take up slack has far more resistance than any published figure, and no amount of correct sizing fixes it.
Can I use flex duct for the main trunk?
Sizing wise the chart will give you a number, but most codes and most sensible practice keep flex to branch runs and use hard pipe for trunks. Long flex trunks are where the sag, the compression, and the extra friction all compound at once.