FabLayout

Rectangular duct on round pipe calculator

For a rectangular duct branch on a round pipe: enter the pipe diameter, the duct length and width, the angle and offset. You get the duct pattern with its fold lines and the hole in the pipe.

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Enter your measurements

Your dimensions are measured to the

Export from the Trueflat app

1:1 PDF and DXF files of this flat pattern are made in Trueflat, ready for print, plasma, laser or waterjet. It works offline on site.

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982.6 mm400.0 mm
Flat pattern: cut on the bold outline, bend on the dashed blue lines, the thin lines are marking lines

Seam

1,667.8mm

Exact geometry

Cut length
In the app
Cut curve length
In the app
Sheet size
In the app

Every result is in Trueflat

The free web version shows the main result and the drawing. Trueflat gives you every mark and measurement, and exports a cut sheet. It is free to download.

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How to lay it out

  1. Draw the duct stretchout

    Draw the mouth line as width, length, width, length of the duct in a row. The seam is on a corner at each end.

  2. Mark the fold lines

    Square a fold line up at the three inside corners.

  3. Mark the cut

    On the two width faces, divide the face, mark each cut point and join them in a curve. On the two length faces, draw the cut as a straight line between the corner points.

  4. Lay out the hole

    On the pipe pattern, mark each hole point by its distance along the pipe and its arc from the top line, and join them.

Good to know

  • FabLayout runs the same code as Trueflat; the result is exact geometry.
  • Source: No published source. Method: each straight line of the branch is intersected with the main surface and both parts are unrolled, developed on the sheet middle; checked by two independent implementations in Trueflat.

Method and formula

The branch is a rectangular duct. Its length A runs along the pipe (at 90°) and its width B runs across it. The duct is cut face by face, and the pattern is one piece folded at the corners.

  • Length faces: they stay the same distance from the centre plane of the pipe, so they meet it along a straight line, √((R − y)(R + y)) from the pipe axis, where y is the distance of the face from that plane.
  • Width faces: each point across the face is at a different distance from the centre plane, so the cut is a curve, highest in the middle when the duct is centred.
  • Hole: unrolled onto the pipe as distance along it and arc from the top line. Its sides along the pipe are straight, where the length faces meet it.

Curve samples here count the divisions on each curved face, from 4 to 90. The pattern is exact. Enter measurements to the middle of the sheet. Which face of the branch seats on the main depends on how you fit and weld it, so this shape is developed on the sheet middle only.

Worked example

Pipe 200 mm, duct 100 × 60 mm, 200 mm long, 90°, centred: the length faces are cut straight at √(100² − 30²) = 95.39 mm from the pipe axis; the width faces rise to 100 mm in the middle. The hole runs from 150 to 250 mm along the pipe, and its corners are 30.47 mm of arc either side of the top line.

Questions

Which duct dimension runs along the pipe?

The duct length. The duct width runs around the pipe and must not be more than the pipe diameter.

How is this different from the round branch pages?

A round branch has one continuous fish mouth. A rectangular duct has two straight cuts, two curved cuts and fold lines at the corners, so it needs its own page.

Where is the seam?

On one corner of the duct, so the pattern starts and ends at a fold. The pipe seam is on the bottom of the pipe.

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