Rectangle to Round flat pattern

What it is

The rectangle-to-round transition swaps a rectangular opening for a circular one. The surface is four flat triangles at the corners of the rectangle, alternating with four conical sectors that carry the curve. Only triangulation solves it: there is no direct formula.

Where it is used

Fan outlet to round duct, hood openings, industrial exhaust transitions, connections between rectangular equipment and circular pipework.

Measurements the calculation needs

• H Height
• D Diameter
• X Measurement X
• Y Measurement Y
• THK Thickness

Worked example

The values the form comes pre-filled with, run through the tool itself:

Height	1000 mm
Diameter	450 mm
Measurement X	1000 mm
Measurement Y	600 mm
Thickness	3 mm
The flat pattern fits a plate of	1400 × 1318 mm

Frequently asked questions

Why is this part considered hard to lay out by hand?

Because each triangular strip has a different true length that does not appear in the drawing views — it has to be rotated into view first. Get one strip wrong and the whole sequence shifts, and the error only shows when the part will not close.

Can the round opening be off-centre?

Not in this part: the round is concentric with the rectangle. For an offset and tilted opening, use the inclined square to round.