If you need to cut a clean saddle notch (also called a fishmouth cut) so one pipe fits tightly against another, this pipe notch calculator gives you practical layout values in seconds. Enter your pipe sizes and joint angle, then use the generated offsets to mark and cut accurately.
Pipe Notch / Fishmouth Calculator
Use consistent units for both diameters (inches or millimeters). Results are returned in the same unit.
What this calculator does
This tool calculates the notch profile formed by the intersection of two cylinders:
- Branch pipe: the pipe you are coping/notching
- Run pipe: the pipe the branch fits into
- Angle: centerline angle between the two pipes
The output gives a list of points around the branch circumference and the cutback depth from the longest point. This is exactly what you need for manual marking, wrap templates, or CNC setup.
How to use the pipe notch calculator
1) Enter your diameters
Use outside diameter values unless your process specifically references nominal sizes with known OD conversion. The calculator assumes round pipe/tube.
2) Set the joint angle
Common values are 90°, 60°, 45°, and sometimes shallow angles for handrails, roll cages, and structural tube frames.
3) Choose the number of points
Higher point counts create a smoother template. A good starting range:
- 24 points for quick shop layout
- 36 to 72 points for precision hand templates
- 120+ points for CAD/CAM workflows
4) Transfer to pipe
Mark your wrap distance increments around the pipe circumference. At each location, measure the cutback depth from the long point, then connect points with a smooth curve and cut.
Understanding the results
- Maximum notch depth: total difference between longest and shortest profile points.
- Template circumference: wrap length of the branch OD.
- Point spacing: distance between neighboring layout marks.
- Cutback from long point: practical shop-friendly value for marking.
Practical fabrication tips
Improve fit-up quality
- Deburr before fitting; burrs can make a perfect notch look wrong.
- Keep saw and grinder square to avoid tapering the notch line.
- Tack at 3 or 4 points, then verify alignment before final weld.
Account for weld gap and process
MIG, TIG, and stick setups tolerate different root gaps. If you intentionally want a small gap, offset your marked line slightly. Always test with a short sample piece when tolerances are tight.
Check angle in fixture, not by eye
A 1-2° angle error can ruin fit-up on tight joints. Use a digital angle finder, fixture stops, or a repeatable jig.
Frequently asked questions
Is this a tube coping calculator too?
Yes. For geometry purposes, this works for round tube and round pipe as long as you enter outside diameters.
Why do I get an invalid geometry warning?
Some diameter and angle combinations do not produce a full 360° intersection profile. This often happens when a large branch meets a much smaller run pipe at certain angles.
Do wall thickness and schedule matter?
For the external notch line, OD drives the geometry. Wall thickness matters for weld prep, penetration strategy, and structural design—not for the outer fishmouth profile itself.
Final notes
This calculator is intended for fabrication planning and shop layout. Verify dimensions, fit-up, and safety requirements before cutting production parts. For critical pressure, structural, or code work, follow applicable standards and qualified procedures.