Reference · Duct dimensions
Standard duct sizes
Round duct comes in 1-inch steps from 4 to 10 inches, then 2-inch steps above that. Rectangular duct is built to order but shops work from a small set of standard depths. Here is what is actually stocked, and what each size carries.
Round duct sizes
Airflow shown at the standard 0.10 in/100 ft design friction rate, galvanized metal.
| Diameter | Carries (CFM) | Typical use |
|---|---|---|
| 4 in | 37 | Small branch, single register |
| 5 in | 67 | Small branch, single register |
| 6 in | 109 | Small branch, single register |
| 7 in | 163 | Branch, small trunk |
| 8 in | 232 | Branch, small trunk |
| 9 in | 317 | Branch, small trunk |
| 10 in | 419 | Branch, small trunk |
| 12 in | 678 | Residential trunk |
| 14 in | 1,019 | Residential trunk |
| 16 in | 1,450 | Residential trunk |
| 18 in | 1,980 | Main trunk, light commercial |
| 20 in | 2,615 | Main trunk, light commercial |
| 22 in | 3,364 | Main trunk, light commercial |
| 24 in | 4,233 | Main trunk, light commercial |
| 26 in | 5,230 | Main trunk, light commercial |
| 28 in | 6,362 | Main trunk, light commercial |
| 30 in | 7,634 | Main trunk, light commercial |
Rectangular duct sizes
Common stocked combinations with their Huebscher equivalent round diameter — the number that decides capacity.
| Size (W × H) | Equivalent ⌀ | Carries (CFM) |
|---|---|---|
| 8 × 6 in | 7.6 in | 200 |
| 10 × 6 in | 8.4 in | 264 |
| 12 × 6 in | 9.1 in | 330 |
| 14 × 6 in | 9.8 in | 397 |
| 10 × 8 in | 9.8 in | 393 |
| 12 × 8 in | 10.7 in | 496 |
| 14 × 8 in | 11.5 in | 600 |
| 16 × 8 in | 12.2 in | 706 |
| 18 × 8 in | 12.9 in | 814 |
| 20 × 8 in | 13.5 in | 922 |
| 12 × 10 in | 12 in | 673 |
| 16 × 10 in | 13.7 in | 969 |
| 20 × 10 in | 15.2 in | 1,274 |
| 24 × 10 in | 16.5 in | 1,585 |
| 18 × 12 in | 16 in | 1,446 |
| 24 × 12 in | 18.3 in | 2,062 |
| 30 × 12 in | 20.2 in | 2,692 |
Sheet metal gauge
| Duct size | Typical gauge |
|---|---|
| Round up to 12 in | 30 – 28 ga |
| Round 14 – 24 in | 26 ga |
| Round above 24 in | 24 ga |
| Rectangular, longest side up to 12 in | 30 – 28 ga |
| Rectangular, longest side 14 – 30 in | 26 ga |
| Rectangular, longest side above 30 in | 24 ga |
These are typical figures for low-pressure residential work. Commercial ductwork follows SMACNA tables, where gauge and reinforcement depend on the pressure class rather than size alone.
FAQ
- What are the standard round duct sizes?
- Round duct is stocked in 1-inch steps from 4 to 10 inches, then in 2-inch steps: 12, 14, 16, 18, 20, 22, 24 and up. Residential branches are usually 5 to 8 inches; trunks run 10 to 20 inches.
- What are standard rectangular duct sizes?
- Rectangular duct is made to order, but shops work in 1-inch increments and stock a common set of depths — 6, 8, 10 and 12 inches are typical for residential trunks, since they fit between joists. The width is then chosen to hit the equivalent round diameter the airflow needs.
- What gauge sheet metal is used for ductwork?
- For typical low-pressure residential work: 30 or 28 gauge for round duct up to about 12 inches, 26 gauge from roughly 14 to 24 inches, and 24 gauge above that. Commercial work follows SMACNA tables, where the gauge depends on the duct's pressure class and reinforcement spacing, not size alone.
- Why are duct sizes not in even inches above 10?
- They are — that is the point of the 2-inch step. Below 10 inches a single inch is a large relative change in area, so the steps are finer; above it, 2-inch steps give a sensible spread of capacities without an unwieldy number of stocked sizes.
- Can I use a rectangular duct with the same area as the round one?
- No. Equal area is not equal capacity: corners add friction, so a rectangular duct always needs more area than the round one it replaces. Use the Huebscher equivalent diameter — the table on this page and the ductulator both apply it.
Size a specific run with the ductulator, or start from your airflow on the duct size chart. Sizing the return side? Return air duct sizing.