Updated September 2026
Round duct is the most efficient shape to move air through — least friction and least sheet metal per CFM — but it doesn't always fit in a joist bay or above a dropped ceiling. Rectangular duct trades some of that efficiency for a flatter profile. The question is: what rectangular size actually carries the same airflow at the same friction rate as a given round duct? That's what this page and the calculator's rectangular table answer.
The standard way to compare a round duct to a rectangular one isn't by cross-sectional area — a rectangle and a circle with the same area don't move air with the same friction, because a rectangle has more perimeter (and therefore more friction) per unit of area than a circle does. Instead, ASHRAE defines an equivalent diameter: the diameter of the round duct that has the same friction characteristics as a given rectangular duct, for the same airflow.
De = 1.30 × (a·b)0.625 / (a+b)0.25
where a and b are the rectangular duct's two side lengths in inches and De is the equivalent round diameter. This equation can't be rearranged cleanly to solve for one side given the other and a target De, so the calculator solves it numerically (bisection search) for each standard height rather than using a lookup table.
Each row below holds the height fixed at common sheet-metal shop increments and solves for the matching width, so the resulting rectangle has the same equivalent diameter — and therefore the same friction and airflow — as the round duct on the left.
| Round duct | Rectangular equivalent | Aspect ratio |
|---|---|---|
| 10" | 11" × 8" | 1.3:1 |
| 12" | 15" × 8" | 1.9:1 |
| 12" | 12" × 10" | 1.2:1 |
| 14" | 22" × 8" | 2.7:1 |
| 14" | 17" × 10" | 1.7:1 |
| 14" | 14" × 12" | 1.1:1 |
| 16" | 30" × 8" (exceeds 4:1) | 3.7:1 |
| 16" | 22" × 10" | 2.2:1 |
| 16" | 18" × 12" | 1.5:1 |
| 16" | 15" × 14" | 1.1:1 |
| 18" | 29" × 10" | 2.9:1 |
| 18" | 23" × 12" | 1.9:1 |
| 18" | 19" × 14" | 1.4:1 |
Widths rounded to the nearest inch. Run any round size through the calculator for the full table including every standard height from 8" to 18".
Notice the 16" round → 30"×8" row is flagged above: at 3.7:1 it's borderline, and going even flatter (say a 6" height) would push it past 4:1 entirely. Very flat, wide ducts have disproportionately more friction from the two long flat sides than the equivalent-diameter formula alone suggests once you get past that ratio in practice, and they're structurally floppier — more prone to oil-canning noise and harder to insulate and support properly. As a working rule, if the flattest option in a table like this exceeds roughly 4:1, step up to the next taller height instead of forcing the flattest one to fit.
There's no single "correct" rectangular size for a given round duct — every row for the same round size in the table above carries identical airflow at identical friction, because they all share the same equivalent diameter. The right choice is whichever height actually fits the joist bay, soffit, or chase you're routing through, picked from tallest (most efficient, closest to round) down to the flattest one that still respects the 4:1 guideline.
If a rectangular duct someone specified looks suspiciously small next to a round duct handling the same air, check the equivalent diameter, not the area. A 12"×8" rectangle has 96 sq in of area — close to a 12" round duct's ~113 sq in — but its equivalent diameter is only about 10.7", meaning it actually carries meaningfully less air at the same friction rate than a straight area comparison would suggest, because of the extra perimeter friction. Area comparisons alone routinely undersize rectangular duct for exactly this reason.
Can I convert rectangular to round instead? Yes — the same formula works either direction. Plug your rectangular duct's two sides into the equivalent-diameter equation to get the round size that carries the same air at the same friction.
Does this account for flex duct? No — like the rest of the site, this is based on smooth galvanized sheet metal, which has meaningfully less friction than flex duct (commonly cited at 3–8× depending on installation quality) — size flex runs up one increment from these figures.
Why isn't my exact round size or height listed? The table above covers common sizes for quick reference. Use the calculator for any round duct size — it generates the full rectangular table on the fly.