Rectangular to Round Duct Converter
Converts rectangular and flat oval duct to circular equivalent diameter using the Huebscher equation, and works in reverse: give it a round duct and a ceiling void depth and it lists rectangular sizes carrying the same air at the same friction rate.
Enter valid positive dimensions, then select Convert.
A design calls for an 18 in round spiral trunk, but the ceiling void above the corridor only allows 12 in of depth. The engineer needs a rectangular size that carries the same air at the same friction rate.
- 1
State the equal-friction requirement
Find W × H with De = 18 in and H ≤ 12 in
Equivalent diameter, not equal area — equal area would undersize the duct.
- 2
Apply the Huebscher equation
De = 1.30 × (W×H)^0.625 / (W+H)^0.25 = 18 → 24 × 12 gives De ≈ 18.4 in
That is the equation this converter runs in both directions.
- 3
Check the aspect ratio and velocity
Aspect = 24/12 = 2:1 — well inside the 4:1 practical limit
Enter the CFM to see real-section velocity, which drives noise, not the equivalent-round figure.
Takeaway: A 24 × 12 fits the void and carries the same air at the same friction — but it uses about a third more sheet metal than the round it replaces, which is worth flagging in the estimate.
This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.