Obround to Rectangle flat pattern

What it is

The transition between a flattened obround mouth and a rectangular one, in parallel planes on the same axis. It is the other end of the round-to-oval job — the flattened duct gets through the low gap and then has to meet the rectangular frame of a filter, a fan or a hood.

Where it is used

Industrial exhaust, connecting flat oval duct to a filter frame, fan mouths, commercial kitchen hoods, anywhere obround meets rectangular.

Measurements the calculation needs

• X Length of the obround (base)
• Y Width of the obround (base)
• X2 Side of the rectangle along x (top)
• Y2 Side of the rectangle along y (top)
• H Height between the mouths
• DV Divisions
• THK Plate thickness

Calculate with your own measurements

No account needed: enter the measurements in millimetres or inches and get the figures and the drawing. The scaled PDF template, the DXF for the cutting table and the 3D model take a free account.

Diameters and sides are outside measurements. The flat pattern already allows for the plate thickness, working on the mean line.

Units: mm inches

Length of the obround (base) X (in)

Width of the obround (base) Y (in)

Side of the rectangle along x (top) X2 (in)

Side of the rectangle along y (top) Y2 (in)

Height between the mouths H (in)

Divisions DV

Plate thickness THK (in)

Calculate free

Worked example

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

Length of the obround (base)	31-1/2" (31.4961)
Width of the obround (base)	11-13/16" (11.811)
Side of the rectangle along x (top)	23-5/8" (23.622)
Side of the rectangle along y (top)	15-3/4" (15.748)
Height between the mouths	19-11/16" (19.685)
Divisions	16
Plate thickness	1/8" (0.1181)
The flat pattern fits a plate of	78-1/8" × 24"

Result of the example

Plate	78-1/8" × 24"
Area and weight	10.6 ft² · ≈ 51.3 lb in 1/8" steel

Frequently asked questions

Does the pattern have creases?

None at all. At both meetings the obround's tangent is parallel to the rectangle side it meets — at the end of the straight run it lies along x, at the tip of the obround along y — and all four flat faces touch the mouth without cutting through it. That is what separates this part from the angular boot, which has two creases and cannot avoid them.

Can the rectangle be larger than the obround?

It can, in both directions at once. There is no guard tying one to the other, and there should not be — with both mouths in parallel planes the tangency points do not move with the rectangle's size, and the part simply flares outward.

Why can't the obround be a circle?

Because then the two flat trapezoids degenerate and the part becomes a different one — the rectangle-to-round, which is already in this catalogue. The guard asks for half a millimetre of straight run per side.

Where does the seam go?

In the middle of one of the straight runs, rising to the middle of the rectangle's long side. That is one whole flat face, so the weld falls on a straight line.