Air distribution · Round duct
How many CFM does a 18 inch duct carry?
A 18-inch round metal duct carries about 1,980 CFM at the standard 0.10 in/100 ft friction rate, with the air moving at 1,120 fpm. Design it quieter at 0.05 and it carries about 1,374 CFM.
Airflow by design friction rate
| Friction (in/100 ft) | CFM | Velocity (fpm) | Typical use |
|---|---|---|---|
| 0.05 | 1,374 | 778 | Long runs, quiet design |
| 0.08 | 1,760 | 996 | Typical trunk |
| 0.10 | 1,980 | 1,120 | Standard residential |
| 0.15 | 2,451 | 1,387 | Short branch |
Velocities above roughly 900 fpm get audible in a home; commercial mains tolerate more.
Rectangular equivalents
Rectangular ducts that match a 18-inch round, by Huebscher equivalent diameter. Only buildable shapes are listed — nothing narrower than 4 inches or beyond a 4:1 aspect ratio.
| Width needed | Equivalent ⌀ |
|---|---|
| 30 × 10 in | 18.3 in |
| 24 × 12 in | 18.3 in |
FAQ
- How many CFM can a 18 inch duct handle?
- About 1,980 CFM at the standard residential design friction rate of 0.10 in. w.c. per 100 ft, moving at 1,120 fpm. Designed quieter at 0.05, the same duct carries about 1,374 CFM.
- Is that figure for flexible duct too?
- No — these numbers are for galvanized sheet metal. Flexible duct has noticeably more resistance, so a 18-inch flex run carries meaningfully less. Either size one diameter up or design the flex portion at a lower friction rate, and keep it pulled tight.
- What size room does that serve?
- At the usual 400 CFM per ton, 1,980 CFM is roughly 4.95 tons of cooling — enough for the branch serving one to two rooms in most homes, depending on the load.
Other sizes: 16 inch · 20 inch · ductulator · full chart