Eccentric Rectangle to Round flat pattern
What it is
The rectangle-to-round transition with the round opening off centre. It is a different part from the concentric one, not a case of it: there all four corners are alike and half a pattern mirrored will do; here every corner has its own true lengths and the pattern has to be walked right around, once.
Where it is used
Connecting misaligned equipment to ductwork, chute and trough outlets, extraction hoods over a bench against a wall, off-axis fan take-offs.
Measurements the calculation needs
- X Side X of the rectangular mouth
- Y Side Y of the rectangular mouth
- D Diameter of the round mouth
- H Height between the mouths
- EX Offset of the round in X
- EY Offset of the round in Y
- 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
Worked example
The values the form comes pre-filled with, run through the tool itself:
| Side X of the rectangular mouth | 39-3/8" (39.3701) |
|---|---|
| Side Y of the rectangular mouth | 23-5/8" (23.622) |
| Diameter of the round mouth | 17-11/16" (17.7165) |
| Height between the mouths | 39-3/8" (39.3701) |
| Offset of the round in X | 7-7/8" (7.874) |
| Offset of the round in Y | 1-15/16" (1.9685) |
| Divisions | 16 |
| Plate thickness | 1/8" (0.1181) |
| The flat pattern fits a plate of | 91-1/4" × 65-1/2" |
Result of the example
| Plate | 91-1/4" × 65-1/2" |
|---|---|
| Area and weight | 25.5 ft² · ≈ 123.1 lb in 1/8" steel |
Guides about this part
- Square to round transition: development by triangulation, step by step
- Eccentric and concentric reducers: which one to use and how each one develops on plate
- Duct transition layout: square to round duct, centred, offset or flat on one side
- Register boot layout: from a floor, wall or ceiling register to a round duct, in inches
- Plenum box and plenum transition layout: the sides, the offset and the round outlet
Frequently asked questions
- Where is the offset measured from?
- From the centre of the rectangular opening to the centre of the round one, split into the two directions. Zero offset in both gives exactly the concentric transition — I checked, and the pattern area matches the concentric part in this same catalogue to four decimal places.
- Why can't the pattern be mirrored like the concentric one?
- Because the symmetry is gone. On the concentric part the four corner fans are the same cone repeated, so half the pattern mirrored gives the other half. Move the circle and they become four different oblique cones: every corner edge has its own true length, and no half serves as the mirror of the other.
- Can the round opening cross the edge of the rectangle?
- No, and the calculation refuses it. Outside the rectangle's footprint the surface would fold into itself — there would be no part left, and the triangles would come out with their apex on the wrong side of the base.
- Do the divisions of the round opening change the pattern?
- They do, and for the better the more there are. The round edge is approximated by segments, and each extra division approximates it more closely. The divisions are taken along the arc and not along the chord, precisely so the opening does not come out short.
You can work out the figures right here, with no account. PDF, DXF, 3D and history take an account — free, with unlimited calculations.