Round vs Rectangular Duct Sizing: Equivalent Diameter Explained
Round duct moves air with less friction and less metal than any rectangular section, but buildings rarely give you the space. This guide explains equivalent diameter, the aspect ratio penalty on pressure drop and cost, and gives a round-to-rectangular lookup table for the sizes you actually specify.
Last updated September 2026
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Equal area is not equal capacity
The most common conversion mistake is matching cross-sectional area. A 20 in round duct has 314 in² of area; a 20" × 16" rectangular duct has 320 in². They look interchangeable and they are not. The rectangular section has roughly 15% more wetted perimeter, so more of the airstream is dragging against metal and the friction rate is higher.
The correct basis is equivalent diameter — the round duct that produces the same friction loss at the same airflow. ASHRAE uses the Huebscher relation:
where a and b are the rectangular sides in inches. Size the round duct first, then find a rectangular section whose De matches it.
What aspect ratio actually costs you
Aspect ratio is the long side divided by the short side. As it rises, the perimeter needed to hold a given equivalent diameter grows fast — and perimeter drives three costs at once:
| Aspect ratio | Sheet metal vs round | Practical effect |
|---|---|---|
| 1:1 | ≈ +12% | Best rectangular case. Use wherever the ceiling allows. |
| 2:1 | ≈ +18% | The commercial default. Modest penalty, fits most plenum spaces. |
| 3:1 | ≈ +27% | Noticeable. Reinforcement and hanger spacing tighten. |
| 4:1 | ≈ +37% | Practical upper limit. Expect higher fan energy for life. |
| 6:1 | ≈ +58% | Avoid. Cost, leakage area and breakout noise all climb. |
Perimeter comparison at matched equivalent diameter; exact figures depend on the size, and the calculator below computes them for your case.
The pressure drop penalty tracks the same curve. Two sections with the same equivalent diameter have the same friction rate by definition — but to hold that De at 4:1 you need a much larger section than at 2:1. If you instead squeeze the duct into the available depth and keep the area constant, De falls, velocity rises and the friction rate climbs steeply. That is the real-world aspect ratio pressure loss: it shows up as a duct that was flattened to fit, not as a duct that was converted correctly.
Round baseline: 1,375 FPM at 0.117 in. w.g./100 ft, 62.8 in of perimeter per foot of duct.
| Aspect ratio | Section | Equivalent Ø | Velocity | Sheet metal vs round |
|---|---|---|---|---|
| 1:1 | 19" × 19" | 20.8 in | 1,197 FPM | +21% |
| 1.5:1 | 24" × 16" | 21.3 in | 1,125 FPM | +27% |
| 2:1 | 28" × 14" | 21.3 in | 1,102 FPM | +34% |
| 3:1 | 33" × 11" | 20.1 in | 1,190 FPM | +40% |
| 4:1 | 40" × 10" | 20.7 in | 1,080 FPM | +59% |
| 6:1 | 54" × 9" | 22.0 in | 889 FPM | +101% |
All sections carry the same airflow at the same equivalent diameter; the extra perimeter is extra metal, extra sealing and extra surface friction.
Round to rectangular duct conversion table
Rectangular sections with matching equivalent diameter, rounded up to whole inches, for common round sizes.
| Round Ø | 1:1 | 2:1 | 3:1 | 4:1 |
|---|---|---|---|---|
| 8 in | 8×8 | 12×6 | 15×5 | 16×4 |
| 10 in | 10×10 | 14×7 | 18×6 | 20×5 |
| 12 in | 11×11 | 16×8 | 21×7 | 24×6 |
| 14 in | 13×13 | 20×10 | 24×8 | 28×7 |
| 16 in | 15×15 | 22×11 | 27×9 | 32×8 |
| 18 in | 17×17 | 24×12 | 30×10 | 36×9 |
| 20 in | 19×19 | 28×14 | 33×11 | 40×10 |
| 24 in | 22×22 | 32×16 | 42×14 | 48×12 |
| 28 in | 26×26 | 38×19 | 48×16 | 56×14 |
| 32 in | 30×30 | 44×22 | 54×18 | 64×16 |
| 36 in | 33×33 | 48×24 | 60×20 | 72×18 |
Widths × heights in inches. Sizes are rounded up, so equivalent diameter is at or slightly above the round duct.
Need a different ratio or the reverse conversion? The rectangular to round converter works both directions and reports velocity and friction rate.
When to choose which
- Round — lowest friction per CFM, least metal, best for leakage class, self-reinforcing. Use it in shafts, mechanical rooms and exposed industrial work.
- Flat oval — most of round's efficiency in a shallower profile. Good compromise for tight plenums on larger trunks.
- Rectangular — the space solution. Keep it near 2:1, reinforce per SMACNA for the pressure class, and seal the longitudinal seams properly, because the added perimeter is also added leakage path.
Frequently asked questions
How do I find the equivalent rectangular duct size for a round duct?
Use the Huebscher equation De = 1.30 x (a x b)^0.625 / (a + b)^0.25, where a and b are the rectangular sides in inches. Choose a width and height whose De matches the round diameter you are replacing.
Does a rectangular duct of equal area carry the same air as a round duct?
No. Equal area is not equal capacity. A rectangular duct has more wetted perimeter than a round duct of the same area, so it has more friction. Match equivalent diameter, not area.
What aspect ratio should I avoid?
Keep aspect ratio at or below 4:1 wherever possible, and prefer 2:1 or lower. Beyond 4:1 the sheet metal cost, reinforcement and friction penalties rise faster than the space you save.