Calculator
Example Data Table
| Run OD | Branch OD | Angle | Points | Allowance | Use Case |
|---|---|---|---|---|---|
| 6 in | 3 in | 90° | 24 | 0.125 in | Standard tee saddle |
| 8 in | 4 in | 60° | 36 | 0.1 in | Angled branch layout |
| 150 mm | 75 mm | 45° | 48 | 2 mm | Metric field template |
Formula Used
The calculator models two round pipes as intersecting cylinders.
Run radius: R = Run OD ÷ 2
Branch radius: r = Branch OD ÷ 2
Branch circumference: C = π × Branch OD
Axial cut coordinate: s = (√(R² - r² sin²φ) - r cosφ cosA) ÷ sinA
Raw template height: H = s - minimum s
Mark height: M = H + allowance + kerf ÷ 2
Here, A is the branch angle from the run pipe axis. The value φ is the point angle around the branch pipe.
How to Use This Calculator
Enter the outside diameter of the run pipe.
Enter the outside diameter of the branch pipe.
Add the branch angle measured from the run pipe axis.
Choose enough template points for a smooth wrap pattern.
Add allowance and kerf values if needed.
Press Calculate to view the result above the form.
Use the table to mark points around the branch pipe.
Download the CSV or PDF report for shop records.
Pipe Saddle Cut Layout Guide
Why Saddle Cuts Matter
A pipe saddle cut helps one round pipe fit against another round pipe. It is also called a fishmouth cut. The shape looks simple, but the layout changes fast. Diameter, angle, allowance, and cutting kerf all affect the final line. A small error can create gaps. Large gaps can make welding slower. They can also weaken the joint. This calculator gives a repeatable point table. It helps fabricators mark the branch pipe before cutting.
Better Field Layout
The main idea is to unwrap the branch pipe into a flat template. Each point has a distance around the pipe. Each point also has a height from a square end. Mark the wrap distance along the circumference. Then mark the height at that station. Connect the points with a smooth curve. The curve becomes the saddle line. More points give a smoother template. Fewer points are faster, but less exact.
Angle and Diameter Effects
A ninety degree joint usually has a balanced saddle. A lower angle stretches the cut. It also creates a longer nose on one side. A large branch pipe creates deeper notches. A smaller branch pipe creates a shallower cut. The calculator checks the geometry with the run radius and branch radius. Use outside diameters when you mark the outside of a pipe. Use consistent units for every input.
Allowances and Cutting Kerf
Real cuts need practical allowance. A saw, grinder, plasma cutter, or torch removes material. That removed width is the kerf. The calculator adds half the kerf to the mark height. It also adds your fit allowance. This gives room for cleanup. Use a small allowance for accurate equipment. Use a larger allowance when hand grinding. Always test the first template on scrap when tolerances are tight.
Safe Fabrication Notes
This tool supports layout planning. It does not replace engineering design. Pipe loads, weld procedures, pressure ratings, and codes still matter. Check drawings before cutting production material. Confirm the branch angle with a reliable square or bevel gauge. Keep the seam location clear. Label the zero point before wrapping the template. After cutting, deburr the edge. Then fit, tack, inspect, and weld as required.
FAQs
What is a pipe saddle cut?
It is a curved cut on one pipe. It lets that pipe fit against another round pipe. The shape is common in welded branches, handrails, frames, and tube fabrication.
Which diameter should I enter?
Use outside diameter when marking the outside surface. Keep both pipe diameters in the same unit. Do not mix inches and millimeters in one calculation.
What does branch angle mean?
It is the angle between the branch pipe axis and the run pipe axis. A straight tee is usually 90 degrees. A sloped branch may be 45 or 60 degrees.
How many template points should I use?
Use 24 points for common work. Use 36 or 48 points for larger pipe or smoother curves. More points improve layout detail, but marking takes longer.
What is seam offset?
Seam offset rotates the template around the branch pipe. Use it when you want the zero point away from a weld seam or reference mark.
Why add cutting kerf?
Cutting tools remove material. Kerf compensation helps place the cut line so the final pipe edge remains closer to the intended saddle shape.
Can this calculator handle metric values?
Yes. Select millimeters or centimeters. The formula works the same. Just keep all diameter, allowance, and kerf values in the chosen unit.
Is this enough for pressure piping?
No. This calculator helps with layout. Pressure piping also needs approved design, code checks, welding procedures, inspection, and qualified fabrication practices.