Rectangular to Round Duct Conversion Chart
The equivalent round diameter for every common rectangular duct size — equal friction and equal airflow, per the ASHRAE Fundamentals equivalent-diameter relation (the same math behind our duct size calculator). Read the matrix for rectangle-to-round, or the second table for the jobsite question in reverse: what rectangle replaces a round size when ceiling depth is fixed. These are friction equivalents, not area equivalents — an equal-area swap under-sizes the rectangle.
Rectangular → equivalent round diameter
| Width | 6" deep | 8" deep | 10" deep | 12" deep | 14" deep | 16" deep | 18" deep | 20" deep | 22" deep | 24" deep |
|---|---|---|---|---|---|---|---|---|---|---|
| 6" | 6.6" | 7.6" | 8.4" | 9.1" | 9.8" | 10.4" | 11" | 11.5" | 12" | 12.4" |
| 8" | 7.6" | 8.7" | 9.8" | 10.7" | 11.5" | 12.2" | 12.9" | 13.5" | 14.1" | 14.6" |
| 10" | 8.4" | 9.8" | 10.9" | 12" | 12.9" | 13.7" | 14.5" | 15.2" | 15.9" | 16.5" |
| 12" | 9.1" | 10.7" | 12" | 13.1" | 14.2" | 15.1" | 16" | 16.8" | 17.6" | 18.3" |
| 14" | 9.8" | 11.5" | 12.9" | 14.2" | 15.3" | 16.4" | 17.3" | 18.2" | 19.1" | 19.9" |
| 16" | 10.4" | 12.2" | 13.7" | 15.1" | 16.4" | 17.5" | 18.5" | 19.5" | 20.4" | 21.3" |
| 18" | 11" | 12.9" | 14.5" | 16" | 17.3" | 18.5" | 19.7" | 20.7" | 21.7" | 22.7" |
| 20" | 11.5" | 13.5" | 15.2" | 16.8" | 18.2" | 19.5" | 20.7" | 21.9" | 22.9" | 23.9" |
| 24" | 12.4" | 14.6" | 16.5" | 18.3" | 19.9" | 21.3" | 22.7" | 23.9" | 25.1" | 26.2" |
| 28" | 13.2" | 15.6" | 17.7" | 19.6" | 21.3" | 22.9" | 24.4" | 25.8" | 27.1" | 28.3" |
| 32" | 14" | 16.5" | 18.8" | 20.8" | 22.7" | 24.4" | 26" | 27.5" | 28.9" | 30.2" |
| 36" | 14.7" | 17.4" | 19.8" | 21.9" | 23.9" | 25.7" | 27.4" | 29" | 30.5" | 32" |
| 40" | 15.3" | 18.2" | 20.7" | 22.9" | 25" | 27" | 28.8" | 30.5" | 32.1" | 33.6" |
| 44" | 15.9" | 18.9" | 21.5" | 23.9" | 26.1" | 28.1" | 30" | 31.8" | 33.5" | 35.1" |
| 48" | 16.5" | 19.6" | 22.3" | 24.8" | 27.1" | 29.2" | 31.2" | 33.1" | 34.9" | 36.6" |
Round → rectangular, at fixed depths
| Round size | 8" deep | 10" deep | 12" deep |
|---|---|---|---|
| 9" | 8.5" wide | — | — |
| 10" | 10.5" wide | — | — |
| 12" | 15.5" wide | 12.1" wide | — |
| 14" | 21.8" wide | 16.7" wide | 13.7" wide |
| 16" | 29.6" wide | 22.3" wide | 18" wide |
| 18" | 39.1" wide | 29" wide | 23.2" wide |
| 20" | — | 37" wide | 29.3" wide |
| 22" | — | 46.3" wide | 36.3" wide |
| 24" | — | — | 44.4" wide |
How to read this chart
Size the round duct first — from a friction-rate target using the duct size calculator or the duct sizing chart — then convert only when the round duct won't fit the space. Find your width down the left edge and your available depth across the top; the cell is the round diameter that rectangle replaces. Going the other way, the second table gives the width you need at the depth you have.
Keep the aspect ratio in check as you trade depth for width: past about 4:1 the duct gets expensive (more metal and hangers per CFM), leaks more perimeter, and its fittings lose more pressure. If the table pushes you past that, the better answers are a deeper chase, two smaller ducts, or flat-oval.
Equal friction, not equal area
The trap in duct conversion is matching cross-sectional area. Air doesn't care about area alone — it cares how much wall it has to rub against, and a rectangle always has more perimeter per square inch than a circle. The ASHRAE equivalent-diameter relation folds that in: it returns the round duct with the same friction loss per foot at the same airflow. That's why an 8 × 8 (64 sq in) only matches an 8.7" round (59 sq in of circle), and why the gap widens as ducts get flatter. Our duct size calculator uses this exact relation for its rectangular results — this chart and the calculator can never disagree.
Common questions
What rectangular duct is equivalent to 12 inch round?
At the common 8" depth, a 15.5" × 8" rectangular duct carries the same air at the same friction loss as a 12" round duct — order 16" × 8". At 10" deep, 12.1" × 10" (order 12" × 10" and it is within a rounding hair). An "equal area" conversion (about 11" × 8") would move noticeably less air.
Why not just match the cross-sectional area?
A rectangular duct has more wall per unit of area than a round duct, so it rubs the airstream harder. Matching areas under-sizes the rectangle: an 8" × 8" duct has more area than an 8" round (64 vs 50 sq in) yet only performs like an 8.7" round, not a 9". The chart uses the ASHRAE equivalent-diameter relation, which matches friction loss and airflow, not area.
What is the formula for equivalent duct diameter?
De = 1.30 × (a·b)^0.625 / (a + b)^0.25, where a and b are the rectangular duct sides. It comes from ASHRAE Fundamentals (the duct-design chapter) and is the basis of every published equivalency table — including this one and the NORDIX duct size calculator.
Does the aspect ratio of a rectangular duct matter?
Yes. The flatter the duct, the more sheet metal, hangers, and friction it takes to move the same air — the equivalent-diameter math accounts for the friction, but cost and fitting losses climb past about 4:1 width-to-depth. The reverse table stops printing pairings beyond 5:1 because at that point you should deepen the duct, run two, or go flat-oval.
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