Flat-Oval to Round Transition Developer

A flat-oval (also called racetrack or stadium-shaped) duct is a rectangle capped by two semicircles - common in HVAC runs that need a duct shallower than round but without a rectangular duct's sharp corners. This tool develops the flat pattern for a straight, centered transition between a flat-oval end and a round end, the fitting needed wherever flat-oval trunk duct connects to round branch duct, a round fan collar, or round flexible duct.

How to use it: enter the flat-oval's overall major (long) and minor (short) dimensions - the minor dimension doubles as the diameter of each rounded end - the round diameter, and the vertical height between the two planes. The tool reports the straight-line distances between corresponding points at the oval's tip and at the middle of its flat side. Which is longer depends on the entered dimensions.

Method: triangulation, split at the tip seam. Both the oval boundary and the round circle are walked from one tip seam to the other in equal steps - equal arc-length steps around the oval's semicircular ends and straight sides, equal angular steps around the circle - and the two open paths are triangulated with the same minimum-diagonal method used for the square-to-round and eccentric reducer developers, then unrolled using true 3D edge lengths. The shape is symmetric about the tip-to-tip axis, so only one half needs to be computed; the other half is its exact mirror.

Worked check. With the defaults (400 mm major, 200 mm minor, 250 mm round diameter, 300 mm height): the tip generatrix is ≈ 309.2 mm - the straight-line distance from the oval tip (200 mm off center) to the matching point on the 125 mm-radius circle, 300 mm below. The flat-side generatrix is shorter, at ≈ 301.0 mm, since the oval's flat side (100 mm off center) sits closer to the round diameter's edge than the tip does.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Flat-oval major (long) dimension

400 mm

Flat-oval minor (short) dimension

200 mm

Round diameter

250 mm

Vertical height

300 mm

Development lines per half

8

Primary result: Generatrix at tip: 309.2 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This is a concentric, centered transition only - the round end shares the flat-oval's centerline and mid-height axis. It does not cover an offset or eccentric flat-oval-to-round transition, a flat-oval-to-rectangle transition, or flat-oval fittings such as elbows, tees and wyes.

This is a faceted triangulated approximation, not an exact development of a smooth transition. Triangle edges and both tip-seam lengths preserve their three-dimensional lengths; curved end boundaries are approximated by chords. More development lines reduce chord error, but do not guarantee a smooth formed fit. Check a paper mock-up and your required dimensions before cutting.

The pattern is the bare developed geometry only - add seam allowance, hem or lap separately, on the outside of the printed outline, before cutting.