Flex Duct CFM Chart
At 0.1 in. w.g. per 100 ft, fully extended, 6 inch flex carries about 93 CFM, 8 inch about 202, 10 inch about 367, and 12 inch about 597. The same diameters in sheet metal carry 111, 241, 439 and 714. Both columns are in the chart below so you can see the gap rather than take a rule of thumb for it.
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Max CFM by flex duct size
Assumptions, so you can check this against whatever you normally use:
- Friction rate 0.1 in. w.g. per 100 ft, equal friction method
- Fully extended flex, absolute roughness 0.003 ft
- Sheet metal column is galvanized, roughness 0.0003 ft, for comparison
- Standard air, 70°F at sea level
- Straight run only. A bend in flex costs far more than the same bend in hard pipe
| Size | Flex CFM |
Sheet metal CFM |
Velocity fpm |
|---|---|---|---|
| 4" | 31 | 37 | 355 |
| 5" | 57 | 68 | 418 |
| 6" | 93 | 111 | 474 |
| 7" | 141 | 168 | 528 |
| 8" | 202 | 241 | 579 |
| 9" | 277 | 331 | 627 |
| 10" | 367 | 439 | 673 |
| 12" | 597 | 714 | 760 |
| 14" | 900 | 1076 | 842 |
| 16" | 1282 | 1534 | 918 audible |
| 18" | 1751 | 2097 | 991 audible |
| 20" | 2314 | 2773 | 1061 audible |
Rows marked audible are over 900 fpm. That is roughly where a branch passing a bedroom starts to be heard in a quiet room, so go up a size on those runs.
What the two columns are telling you
Flex has an absolute roughness around 10 times that of galvanized steel, because the liner rides over a wire helix instead of sitting flat. Through the same diameter at the same friction rate that works out to roughly 16 percent less air. That is the entire content of the gap between the two columns, and it is why swapping a run of hard pipe for the same size flex is a downgrade rather than a wash.
Read it the other way when you are sizing rather than checking: to move the air a given size of metal moves, flex needs to be one size up, sometimes two. What the chart cannot account for is slack. Fully extended is the published condition and it is the only condition these numbers describe.
Working the other direction, from a room's airflow to a size? The flexible duct sizing chart starts from CFM and gives you the size. This page starts from the size you already have.
Where these numbers come from
Every value is calculated, not copied from a supplier sheet. Friction comes from the Darcy-Weisbach equation with the Haaland friction factor, the explicit form of Colebrook used in ASHRAE Fundamentals Chapter 34. For each size the airflow shown is the largest that stays at or under 0.1 in. w.g. per 100 ft, rounded down.
Rounded down, not to the nearest, on purpose. This is a capacity limit. A number rounded up is a number that puts the run over the line, and the whole point of the column is to keep it under.
These charts are free and always will be. If you also need to turn the job into a signable quote and an invoice, that is the paid tool this site pays for itself with. See what it costs.
Flex duct CFM questions
How many CFM will a 6 inch flex duct carry?
About 93 CFM at the standard 0.1 in. w.g. per 100 ft friction rate, pulled fully tight. The same 6 inch in sheet metal carries about 111. Flex is rougher, so the same pipe moves less air.
How many CFM can an 8 inch flex duct handle?
About 202 CFM fully extended, against 241 CFM for 8 inch sheet metal. That is roughly 16 percent less air through the same diameter, which is the number to keep in your head when someone swaps hard pipe for flex on site.
How much air does a 12 inch flex duct move?
About 597 CFM at 0.1 in. w.g. per 100 ft, at 760 fpm. Twelve inch is usually where flex stops making sense as a branch and the run wants hard pipe instead.
Can I push more CFM through flex than the chart says?
Physically yes, and the chart shows what it costs. Every number here sits at 0.1 in. w.g. per 100 ft. Push past it and friction and noise both climb faster than the airflow does, so you spend static pressure you needed elsewhere and the room gets loud.
Why does my flex duct move less air than this chart says?
Almost always slack. These numbers describe fully extended flex. Flex with a few feet of coil left at the boot, or sagging between straps, has resistance well beyond any published figure, and it does not show up on a walkthrough because it is above the ceiling.