Eccentric & Concentric Reducer Layout Developer

A reducer steps a duct or pipe from one diameter to another. A concentric reducerkeeps both circles on the same centerline, so it shortens evenly all the way around; an eccentric reducer shifts the top circle to one side so that one side of the fitting stays flush and straight - the shape needed at a level floor, a level pipe invert, or anywhere a duct or pipe run cannot be allowed to step up on one side. This tool develops the flat (unrolled) pattern for either case from four inputs, and exports a printable, mirror-matched two-half template.

How to use it: enter the larger (bottom) and smaller (top) diameters, the vertical height between them, and the offset between their centers. Leave offset at 0 for a concentric reducer (both centers aligned), or set it up to bottom radius − top radius for a fully eccentric reducer with one perfectly straight, vertical side - the most common eccentric reducer style.

Method: triangulation, split at the offset axis. Both circles are divided into equal angular steps from the near seam (nearest the offset direction) to the far seam (180° around), and the two open arcs are triangulated with the same minimum-diagonal method used for the square-to-round developer, then unrolled using true 3D edge lengths. Because the shape is symmetric about the offset axis, the second half is an exact mirror of the first - only one half needs to be computed.

Worked check. With the defaults (250 mm bottom, 150 mm top, 200 mm height): bottom radius 125 mm − top radius 75 mm = 50 mm, which is exactly the entered offset - so this is a fully flush reducer, and the near side comes out to exactly the height, 200 mm (the shortest a straight side can be between the two planes). The far side is longer, at ≈ 223.6 mm. Lower the offset toward 0 and both sides converge to the concentric value, ≈ 206.2 mm, matching the cone flat pattern tool for the same non-offset dimensions.

Reading the result. Near-side and far-side lengths are the two straight generatrices at the seam ends of each half - use them to check the template against a tape measure before cutting. "Fully flush" confirms the near side landed exactly vertical (offset = bottom radius − top radius); if it reads "No" with a nonzero offset, the fitting is eccentric but one side will still slope slightly, which is expected for any offset short of the maximum.

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.

Larger (bottom) diameter

250 mm

Smaller (top) diameter

150 mm

Vertical height

200 mm

Offset

50 mm

Development lines per half

6

Primary result: Near (short) side length: 200.0 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 faceted triangulated approximation. Both seam edges preserve their three-dimensional lengths; circular 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 fitting 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.