Offset trouser piece calculator
Like the equal-leg trouser piece, but both leg mouths are shifted together by an offset in a chosen direction. Enter the sizes, the offset and its direction to get both leg patterns.
Enter your measurements
Seam
3,091.3mm
Approximate, within ±2.0 mm
- Cut 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.
How to lay it out
Place the mouths in plan
Draw the main pipe circle, then the two leg mouths at ±c ÷ 2 and shift both by the offset e in direction ψ.
Divide each leg
For each leg, divide its seat on the main pipe and its crotch arc into equal parts (each leg has its own seat angle), and the leg mouth at the same angles.
Mark the crotch points
Draw the generators and mark where each crotch-side generator meets the crotch plane between the legs.
Triangulate each leg
Find the true lengths of generators and diagonals and lay out the triangles from the outer seam line round the leg.
Cut both legs
Join the edges with smooth curves. Lay out and mark each leg separately: they are mirror images only when the offset is square to the line of the legs.
Good to know
- This pattern is approximate; the result card shows how close it is.
- Source: Derived from first principles: each leg is the oblique cone between the main-pipe circle and the leg mouth, the crotch lies in a plane, so every facet is flat and the leg is laid out by triangulation on the middle of the sheet. Checked against an independent reference implementation in Trueflat.
Method and formula
This is the two-leg trouser piece with both leg mouths moved together by an offset e in the plan direction ψ. ψ = 0° moves them along the line through the legs, and ψ = 90° moves them square to it. Use it when the outlet pipes must sit off the main pipe centre, for example to clear a wall.
Each leg is still a slanting cone from the main pipe circle to its mouth. Because both mouths move by the same amount, the two cones still meet on one plane, and each leg still sits on its own half of the main pipe. With ψ = 90° that plane stays the centre plane and the two legs are mirror images with the same area. With an offset along the line of the legs the plane tilts and the two patterns differ, so each leg is drawn separately.
- Offset limit: e ≤ (Dg − d) ÷ 2. At that limit the legs are flush with the main pipe on one side; beyond it the part would need a second offset.
- Leg axis: the true lean of each leg, which grows with the offset.
The facets are flat, so the triangulated layout is exact apart from drawing the curves with chords. The mouth is short by at most r(w³ + (π − w)³) ÷ (12n²), where w is half the angle of the leg's seat on the main pipe (90° here, which gives π³r ÷ (48n²)). Inside or outside sizes are converted approximately and rejected above 1 mm of mouth deviation.
Worked example
Dg = 400 mm, d = 200 mm, H = 400 mm, c = 400 mm, e = 50 mm, ψ = 90°, n = 4. Each leg axis leans 27.27°, against 26.57° without the offset. The leg mouths clear the crotch plane by 100 mm and the shortest generator is 167.47 mm. Each leg has an area of 290 986.64 mm², and the two patterns are mirror images.
Questions
When do I use the offset version?
When the two outlet pipes have to sit off the main pipe centre line, for example to keep clear of a wall or to line up with existing pipes. With e = 0 it is the same as the equal-leg trouser piece.
How far can I offset?
Up to (Dg − d) ÷ 2. At that value the legs are flush with the main pipe on one side. A larger offset needs a separate offset piece.
Is the pattern exact?
Nearly. The facets are flat, so the triangulated layout has no twist. Drawing the curves with chords leaves the mouth short by at most π³r ÷ (48n²) here, about 4 mm with a 200 mm leg at n = 4. The crotch curve has no proven bound but shrinks at the same rate.
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