Round to rectangular duct conversion chart
A lookup table for the conversion that comes up on every coordination drawing: a round duct has been sized, and the ceiling void will only take a rectangular one. Find the round diameter on the left, the depth you can fit across the top, and read off the width that carries the same air at the same friction rate.
How to read it: find the round diameter in the left column and the depth the void allows across the top. The cell is width × depth. Widths are rounded up to the next 2 in or 50 mm. Shaded cells sit above a 4:1 aspect ratio; a dash means the depth is already deeper than the duct needs.
Imperial — inches
| Round ø | 6 in deep | 8 in deep | 10 in deep | 12 in deep | 14 in deep | 16 in deep | 18 in deep | 20 in deep | 24 in deep |
|---|---|---|---|---|---|---|---|---|---|
| 6 in | 6 × 6 | — | — | — | — | — | — | — | — |
| 7 in | 8 × 6 | — | — | — | — | — | — | — | — |
| 8 in | 10 × 6 | 8 × 8 | — | — | — | — | — | — | — |
| 9 in | 12 × 6 | 10 × 8 | — | — | — | — | — | — | — |
| 10 in | 16 × 6 | 12 × 8 | 10 × 10 | — | — | — | — | — | — |
| 12 in | 24 × 6 | 16 × 8 | 14 × 10 | 12 × 12 | — | — | — | — | — |
| 14 in | 34 × 6 | 22 × 8 | 18 × 10 | 14 × 12 | — | — | — | — | — |
| 16 in | 46 × 6 | 30 × 8 | 24 × 10 | 20 × 12 | 16 × 14 | — | — | — | — |
| 18 in | 60 × 6 | 40 × 8 | 30 × 10 | 24 × 12 | 20 × 14 | 18 × 16 | — | — | — |
| 20 in | 78 × 6 | 52 × 8 | 38 × 10 | 30 × 12 | 26 × 14 | 22 × 16 | 20 × 18 | — | — |
| 22 in | 100 × 6 | 64 × 8 | 48 × 10 | 38 × 12 | 30 × 14 | 26 × 16 | 24 × 18 | 22 × 20 | — |
| 24 in | 126 × 6 | 80 × 8 | 58 × 10 | 46 × 12 | 38 × 14 | 32 × 16 | 28 × 18 | 26 × 20 | — |
| 26 in | 154 × 6 | 98 × 8 | 70 × 10 | 54 × 12 | 44 × 14 | 38 × 16 | 34 × 18 | 30 × 20 | 24 × 24 |
| 28 in | — | 118 × 8 | 84 × 10 | 66 × 12 | 52 × 14 | 44 × 16 | 38 × 18 | 34 × 20 | 28 × 24 |
| 30 in | — | 140 × 8 | 100 × 10 | 76 × 12 | 62 × 14 | 52 × 16 | 44 × 18 | 40 × 20 | 32 × 24 |
| 32 in | — | — | 118 × 10 | 90 × 12 | 72 × 14 | 60 × 16 | 52 × 18 | 46 × 20 | 38 × 24 |
| 36 in | — | — | 158 × 10 | 120 × 12 | 96 × 14 | 80 × 16 | 68 × 18 | 60 × 20 | 48 × 24 |
| 40 in | — | — | — | 156 × 12 | 124 × 14 | 102 × 16 | 86 × 18 | 74 × 20 | 60 × 24 |
Metric — millimetres
| Round ø | 150 mm deep | 200 mm deep | 250 mm deep | 300 mm deep | 350 mm deep | 400 mm deep | 500 mm deep | 600 mm deep |
|---|---|---|---|---|---|---|---|---|
| 160 mm | 150 × 150 | — | — | — | — | — | — | — |
| 200 mm | 250 × 150 | 200 × 200 | — | — | — | — | — | — |
| 250 mm | 400 × 150 | 300 × 200 | 250 × 250 | — | — | — | — | — |
| 315 mm | 650 × 150 | 450 × 200 | 350 × 250 | 300 × 300 | — | — | — | — |
| 355 mm | 850 × 150 | 600 × 200 | 450 × 250 | 400 × 300 | 350 × 350 | — | — | — |
| 400 mm | 1,150 × 150 | 750 × 200 | 600 × 250 | 500 × 300 | 400 × 350 | — | — | — |
| 450 mm | 1,500 × 150 | 1,000 × 200 | 750 × 250 | 600 × 300 | 500 × 350 | 450 × 400 | — | — |
| 500 mm | 1,950 × 150 | 1,300 × 200 | 950 × 250 | 750 × 300 | 650 × 350 | 550 × 400 | — | — |
| 560 mm | 2,600 × 150 | 1,700 × 200 | 1,250 × 250 | 950 × 300 | 800 × 350 | 700 × 400 | 550 × 500 | — |
| 630 mm | 3,550 × 150 | 2,250 × 200 | 1,650 × 250 | 1,250 × 300 | 1,050 × 350 | 900 × 400 | 700 × 500 | 600 × 600 |
| 710 mm | — | 3,050 × 200 | 2,200 × 250 | 1,700 × 300 | 1,350 × 350 | 1,150 × 400 | 900 × 500 | 750 × 600 |
| 800 mm | — | — | 2,950 × 250 | 2,250 × 300 | 1,800 × 350 | 1,500 × 400 | 1,150 × 500 | 950 × 600 |
| 900 mm | — | — | 3,950 × 250 | 3,000 × 300 | 2,400 × 350 | 2,000 × 400 | 1,500 × 500 | 1,200 × 600 |
| 1000 mm | — | — | — | 3,900 × 300 | 3,100 × 350 | 2,550 × 400 | 1,850 × 500 | 1,500 × 600 |
Why a round duct turns into a rectangle
Round duct is the efficient shape — the least perimeter for the area it encloses, so the least metal and the least friction for the air it carries. It is usually what a duct is sized as. Then the coordination drawing arrives, the ceiling void turns out to be 300 mm deep under the beams, and a 450 mm round duct has nowhere to go.
The conversion is not done on area. A rectangle of the same area as the round duct would carry the same air at the same velocity, but it would lose more pressure, because it has more wall for the air to rub against. The equivalence that duct design uses is equal friction at equal airflow, given by the Huebscher equation, and it always produces a rectangular duct with more area than the round one it replaces.
The equation behind the table
- Huebscher: De = 1.30 × (a × b)0.625 ÷ (a + b)0.250
- Solving for width: with the depth b fixed, find the width a that makes De equal to the round diameter, then round up to the next 2 in or 50 mm
- Aspect ratio: a ÷ b, flagged above 4:1
Worked check: a 24 × 12 inch duct has an equivalent diameter of 1.30 × 2880.625 ÷ 360.25 = 18.28 in, so an 18 inch round duct converted at 12 inches deep needs a width just under 24 in — which is exactly what the table shows after rounding.
Things to keep in mind
- Internal dimensions. The table is for the clear air path. Internally lined duct needs the lining added to both dimensions.
- Velocity drops in the rectangle. Because the area grows, the real velocity in the rectangular duct is lower than in the round one. That is helpful for noise, but use the real figure for terminal selection.
- Depth is the expensive dimension to get wrong. Every inch taken off the depth adds disproportionately to the width. Check whether a local beam drop can be avoided before accepting a 5:1 duct for the whole run.
- Flat oval is the middle option. Where the void is shallow but friction matters, flat oval keeps most of round duct's efficiency. The shape converter handles oval too.
Frequently asked questions
How do you convert a round duct to rectangular?
Find the rectangular section with the same circular equivalent diameter, using the Huebscher equation De = 1.30 × (ab)^0.625 ÷ (a + b)^0.25. In practice you fix the depth the ceiling void allows and solve for the width. The table on this page has already done that for the common sizes; the shape converter does it for any size.
What rectangular duct is equal to a 12 inch round duct?
At 8 inches deep, a 16 × 8 duct after rounding up to the next 2 inches; at 10 inches deep, a 14 × 10; at 6 inches deep, a 24 × 6 — which is past the 4:1 aspect ratio most specifications allow. Exact equivalents come out at in-between widths, so the table rounds them up to a buildable size.
Why is the rectangular duct bigger in area than the round one?
Because a rectangle has more perimeter for the area it encloses, and perimeter is where friction happens. To lose the same pressure per foot while carrying the same air, the rectangular duct needs more area than the round duct — typically 10 to 20 per cent more at sensible aspect ratios, and more again as it gets flatter.
Is equivalent diameter the same as equal area?
No. Equal area would give the same velocity but more friction in the rectangular duct. The circular equivalent diameter used in this chart is defined on equal friction and equal airflow, which is the basis the duct design method needs. For noise and velocity checks, always use the real rectangular area.
Why are some cells shaded?
They are above a 4:1 aspect ratio. Those sizes match the round duct on friction, but the flat sides need more metal, more insulation, more reinforcement and more hangers, and many specifications cap the ratio at 4:1. Where the void forces a shaded size, consider two smaller ducts side by side or a flat oval section.
Why does the table round up instead of to the nearest size?
So that the rectangular duct is never smaller than the round one it replaces. Rounding to the nearest increment can land a few per cent under the required equivalent diameter, which shows up as extra pressure drop on the index run. The shape converter shows the nearest-size match and its error if you want to trade that off deliberately.
Last updated: 24 October 2026