Pipe Notching Calculator – Saddle & Pipe Notch Dimensions
Generate pipe saddle notch dimensions for a centered perpendicular pipe intersection. Enter the run and branch pipe diameters to calculate circumferential cutback points, maximum notch depth and fabrication coordinates.
How the saddle notch is calculated
Branch pipe circumference · each angular position produces a different axial cutback
- Run pipe — the larger pipe that the branch intersects.
- Branch pipe — the pipe being notched.
- Circumferential angle — position around the branch pipe.
- Cutback — distance from the branch's tangent reference plane to the saddle intersection.
- Maximum cutback — the deepest calculated notch position for the selected geometry.
The calculation assumes the two pipe centerlines intersect at 90° and are centered on the same intersection point. The branch pipe is treated as a cylinder intersecting the outside surface of the run pipe.
Fabrication aid only. Actual fit-up depends on pipe outside-diameter tolerance, wall thickness, weld gap, cutting kerf, bevel angle, squareness and the actual geometry of the joint. Verify the cut on the real material before production.
Pipe saddle geometry
Coordinate relationship
x = √(R² − r² sin²φ)
R = run-pipe outside radius · r = branch-pipe outside radius · φ = circumferential angle around the branch.
Cutback
cutback = R − √(R² − r² sin²φ)
The cutback is measured from the branch tangent reference plane used by this calculator.
Symmetry
The centered 90° saddle is symmetric. Points 180° apart have the same cutback, and the maximum cutback occurs at the branch sides where the circumferential sine term reaches its maximum.
Generated notch layout
Run the calculator to generate the circumferential cutback table.
Practical pipe notching tips
- Measure actual outside diameter when tight fit-up matters; nominal pipe size is not the same as outside diameter.
- Mark a consistent zero reference line along the branch before transferring the calculated points.
- Use enough layout points for the required accuracy; more points produce a smoother manual template.
- Keep the cutting kerf outside the finished cut line where possible so the final pipe is not undersized.
- Check the physical fit before welding because a calculated surface intersection does not automatically account for weld root gap or bevel preparation.
- For compound joints, offset intersections, angled branches and complex structural tube joints, use a geometry method specifically designed for those conditions.
What the cutback pattern means
0° position
Reference cutback
90° position
Maximum cutback
180° position
Reference cutback
270° position
Maximum cutback
For a symmetric perpendicular saddle, the cut profile repeats every 180°. This makes the generated table useful for checking manually transferred layout marks.
Frequently Asked Questions
What is a pipe saddle notch?
It is a curved fishmouth or coping cut on the end of one pipe so it fits around another cylindrical pipe.
What does this calculator assume?
It assumes a centered 90° intersection between two circular pipes and calculates the branch cutback from their outside diameters.
Where is maximum cutback?
For this centered geometry, maximum cutback occurs at the side positions of the branch circumference, around 90° and 270° from the reference line.
How many layout points should I use?
24 points provide a useful general-purpose layout. 36, 48 or 72 points can provide a smoother manual approximation for larger or more demanding fabrication work.
Can I use nominal pipe sizes?
Use actual outside diameters when possible. Nominal pipe size can differ significantly from actual outside diameter depending on the pipe standard.
Does the calculator account for weld gap?
An optional fit-up allowance can be entered, but actual weld preparation and root-gap requirements should be determined separately for the joint and welding procedure.