DuctCalc Flex duct friction loss compared with galvanized and duct board
At the same size and airflow, stretched flex loses about twice the pressure of galvanized steel. A 6 in duct at 100 CFM runs at 0.083 in.wg/100ft in galvanized, 0.114 in duct board and 0.162 in flex.
Roughness is the only thing that changes between the three materials, and it changes a lot. DuctCalc takes the absolute roughness from the ASHRAE categories: 0.0003 ft for galvanized, 0.003 ft for duct board and 0.01 ft for flex.
Same duct, three materials
Open Check a duct, choose Round duct, enter 100 for Airflow and 6 for Diameter, then flip the Material control. The velocity stays at 509 fpm and only the friction moves.
| Material | Friction rate |
|---|---|
| Galv. steel | 0.083 in.wg/100ft |
| Duct board | 0.114 in.wg/100ft |
| Flex duct | 0.162 in.wg/100ft |
Same airflow, three sizes
Now go back to Size a duct, type 400 in Airflow and keep Friction rate at 0.1:
| Material | Exact round | Velocity | Stock size |
|---|---|---|---|
| Galv. steel | 9.7 in | 781 fpm | 10 in |
| Duct board | 10.3 in | 687 fpm | 12 in |
| Flex duct | 11.0 in | 606 fpm | 12 in |
Flex needs a bigger duct for the same friction, and the stock line lands on 12 in for both duct board and flex. That jump from 10 to 12 is the real cost of the material change.
What size flex duct for 400 CFM?
At 0.1 in.wg/100ft, 11.0 in exact, so 12 in flex. A 10 in flex would run the same airflow at 733 fpm and 0.166 in.wg/100ft, roughly two thirds more friction than you designed for.
What the flex number assumes
The app treats flex as fully stretched. The label in the Reference tab says so: Flex duct (stretched). Compressed flex, with sags and tight bends, is far rougher, and the app's own footer says no calculator can rescue it. If the run will be installed slack, size up and keep it short.
Questions and answers
What is the friction factor of flex duct?
Is there a spiral duct option?
Does a longer flex run change the answer?
More DuctCalc guides
- How to calculate duct size from CFM
- Round to rectangular duct equivalent, with real velocities
- Recommended duct velocity: the chart, and checking a real duct
- CFM per ton: the chart, and the duct size that carries it
- How many CFM can a 6 inch duct carry?
- What is friction rate in ductwork, and what does a ductulator do with it?
- All questions about DuctCalc
- Back to DuctCalc