Pipe Saddle Template and Fish Mouth Layout Calculator

What is a saddle, fish mouth or lateral cut?

When a branch sits on the outside of a header, the end of the branch has to be cut to the curve of the header. At 90° that cut is a saddle, or fish mouth. At a shallower angle it becomes a lateral, with a long heel on one side and a short throat on the other. Grinding a fish mouth to a tight fit beats freehanding it from eye — 16 stations beats guessing every time.

How are the saddle ordinates worked out?

R = header OD ÷ 2, r = branch OD ÷ 2, φ = the station angle round the branch.
Saddle (90°): reach x(φ) = √(R² − r² cos² φ).
Lateral at angle β: t(φ) = [√(R² − r² cos² φ) − r sin φ cos β] ÷ sin β.
Cutback = longest reach − each reach, at 16 stations. Station 1 is always the longest.

How do you get the template in FitterMaster?

Open Saddle & Lateral under Templates (Pro).
Under Pipes, pick the Header (run) size and the Branch size.
Under Fit, set Branch Angle β: 90° for a saddle, 30° to 89° for a lateral.
Read the Wrap Template: 16 numbered cutbacks and a profile drawing of the cut.
If you see Branch must be ≤ header OD., the branch is larger than the header; swap the sizes.

Example: 4" branch on a 6" header at 90°

Wrap Template, 4" (OD 4.500") on 6" (OD 6.625"), β = 90°
Station	1	2	3	4	5	6	7	8
Cutback	0"	1/8"	7/16"	3/4"	7/8"	3/4"	7/16"	1/8"

Stations 9 to 16 repeat the same pattern: a saddle has two high points and two low points. The deepest cutback, at stations 5 and 13, is 7/8" (0.8814").

Size on size, 4" on 4", the deepest cutback grows to 2-1/4" at stations 5 and 13, because the branch wraps right round the header.

Example: 45° lateral, same pipes

Wrap Template, 4" on 6", β = 45°
Station	Cutback	Station	Cutback
1	0"	9	4-1/2"
2	5/16"	10	4-1/2"
3	1-1/4"	11	4-7/16"
4	2-7/16"	12	4-1/8"
5	3-1/2"	13	3-1/2"
6	4-1/8"	14	2-7/16"
7	4-7/16"	15	1-1/4"
8	4-1/2"	16	5/16"

Station 1 is the heel, the long point; the stations around 9 form the throat.

Laying it out on the branch

Wrap a strip round the branch and divide it into 16 equal spaces, numbering from station 1.
Scribe a square line round the pipe, then mark each cutback back from that line and join the points.
Cut and bevel to your procedure. The app gives the fit-up line, not a bevel angle.
It covers saddle-on branches only: no set-in (saddle-in) cut, no hole template for the header and no off-center branches.

Questions and answers

How do you make a saddle template for pipe?

Work out the reach at 16 stations round the branch from the two ODs, subtract each from the longest to get cutbacks, and transfer them to the pipe with a wrap-around. FitterMaster does the math; you do the layout.

Does it use nominal size or real OD?

Real OD. A 6" header is calculated at 6.625" and a 4" branch at 4.500".

What lateral angles can it do?

Any whole angle from 30° to 89°, and 90° for a straight saddle.

Can the branch be the same size as the header?

Yes. A size-on-size branch is allowed; only a branch bigger than the header is refused.

Pipe schedule chart with ID and weight per foot for carbon-steel pipe · FitterMaster

Pipe saddle template calculator: fish mouth and 45 degree lateral layout