Flex Duct vs Rigid Metal Duct Sizing: Friction Rate Adjustments
Flexible duct is not galvanized steel with a different jacket. Its corrugated inner liner is an order of magnitude rougher, and technicians who size flex off a sheet metal ductulator quietly undersize every runout on the job. Here is what the friction rate should be, how much to upsize, and what compression does to the numbers.
Last updated September 2026
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Why flex duct is not sheet metal
Friction loss in a duct depends on wall roughness through the Darcy-Weisbach equation and the Altshul-Tsal friction factor. The relevant input is absolute roughness, ε:
| Duct material | Absolute roughness ε | Relative to spiral |
|---|---|---|
| Galvanized steel, spiral seam | 0.09 mm | 1× |
| Galvanized steel, longitudinal seam | 0.15 mm | 1.7× |
| Fibrous glass liner, air side | 1.5 mm | 17× |
| Flexible duct, fully extended | 3.0 mm | 33× |
| Flexible duct, 70% extended | 6.0 mm | 67× |
Roughness does not translate one-for-one into pressure drop — the friction factor grows far more slowly than ε — but a 33× roughness increase still lands as roughly two to three times the friction rate at typical runout sizes and velocities.
The friction rate adjustment
The practical fix is to design the flex portions of the system at a lower allowable friction rate than the metal. If your trunk system is sized at 0.10 in. w.g. per 100 ft, budget the flex runouts at about 0.06 in. w.g. per 100 ft. That single change forces the sizing routine to pick the larger duct that flex actually needs.
| System design rate (metal) | Use this rate for flex runouts | Typical result |
|---|---|---|
| 0.08 in. w.g./100 ft | 0.05 in. w.g./100 ft | One standard size up |
| 0.10 in. w.g./100 ft | 0.06 in. w.g./100 ft | One standard size up |
| 0.12 in. w.g./100 ft | 0.07 in. w.g./100 ft | One to two sizes up |
In the duct sizer, the cleaner way to do this is to leave the friction rate alone and change the material to "Flexible duct, fully extended" — the roughness is handled in the physics rather than fudged in the target.
Velocity in this size: 733 FPM.
| Duct type | Friction rate | Loss over the run | vs sheet metal |
|---|---|---|---|
| Galvanized spiral | 0.086 in. w.g./100 ft | 0.022 in. w.g. | baseline |
| Flex, fully extended | 0.149 in. w.g./100 ft | 0.037 in. w.g. | 1.7× |
| Flex, 70% extended | 0.175 in. w.g./100 ft | 0.044 in. w.g. | 2.0× |
To match the sheet metal friction rate with fully extended flex at this airflow, use 12 in flex instead of 10 in.
Flex duct sizing chart
Common runout cases: the friction rate the metal size would produce, what the same size costs you in flex, and the flex size that restores the original friction rate.
| Airflow | Metal size | Metal rate (in. w.g./100 ft) | Same size in flex | Flex size needed |
|---|---|---|---|---|
| 100 CFM | 6 in | 0.084 | 0.137 | 7 in |
| 150 CFM | 7 in | 0.083 | 0.137 | 8 in |
| 200 CFM | 8 in | 0.073 | 0.121 | 9 in |
| 300 CFM | 9 in | 0.085 | 0.146 | 10 in |
| 400 CFM | 10 in | 0.086 | 0.149 | 12 in |
| 600 CFM | 12 in | 0.074 | 0.128 | 14 in |
| 800 CFM | 14 in | 0.059 | 0.102 | 16 in |
| 1,200 CFM | 16 in | 0.064 | 0.113 | 18 in |
Standard air, fully extended flex at ε 3.0 mm, galvanized spiral at ε 0.09 mm.
Compression is worse than sizing
Every one of the numbers above assumes the flex is pulled tight. Flex delivered in a 25 ft box and installed across a 20 ft run is compressed, and compressed flex behaves like a series of sharp bellows. At roughly 70% extension the friction rate can be four times the metal equivalent — worse than any sizing error you could make.
- Cut flex to length and stretch it; do not store the slack in the ceiling.
- Support at 4 ft maximum with 1½ in wide straps, with no more than ½ in sag per foot.
- Keep bends to a centerline radius of at least one duct diameter.
- Limit flex to the runout — typically 5 to 8 ft — and run the rest in metal.
Frequently asked questions
How much more pressure drop does flex duct have than sheet metal?
Fully stretched flexible duct typically loses roughly two to three times the pressure of galvanized spiral duct at the same size and airflow. Compressed to about 70% extension, the loss can be four times or more.
What friction rate should I use for flex duct?
Do not reuse the sheet metal friction rate. If the system is designed at 0.10 in. w.g./100 ft for metal, budget the flex runouts at roughly 0.06 in. w.g./100 ft of allowable rate, which forces the next size up.
Is it enough to upsize flex duct by one size?
One standard size up covers fully extended flex in most runouts. Compressed or sagging flex can need two sizes, and even then the fix is to support and stretch the duct rather than upsize around the problem.