45 degree pipe offset formula: from travel to the pipe you actually cut

Why isn't the travel the cut length?

The travel is measured from the center of one fitting to the center of the other. Each ell eats part of that distance between its center and its welding face, which is the takeout. The weld gap you leave at each joint takes a little more.

Forget the takeouts and the spool comes out too long by twice the center-to-face: on 4" LR 45s that's more than 5". That exact mistake is a common reason a spool gets sent back from the shop on a 6" run — it's an easy one to make in your head, which is the whole reason this calculator exists.

What is the pipe offset formula?

1. Travel = offset × multiplier. The multiplier is 1 ÷ sin(angle); for 45° it is 1.4142.
2. Look up the takeout (center-to-face) of your fitting at that size.
3. Subtract two takeouts and two weld gaps from the travel to get the length you actually cut.

Offset multiplier chart

Worked example: 12" offset on 4" 45s

The same example in FitterMaster: open Offset, enter 1'-0" in Set (rise), leave Roll (0 = simple) at 0, keep 45°, choose Size 4" and Series LR, and set Weld Gap (per joint) to 1/8". The Cut List shows Travel (c-to-c) 1'-5", Takeout (each) 2-1/2" and Cut Length 11-11/16".

A metric case, with Pro: a 400 mm set on 2" LR 45s with a 1/8" gap gives 565.5 mm travel, a 35.0 mm takeout and a 489.0 mm cut.

Where does the 45° takeout come from?

For 45° long-radius ells the app uses the ASME B16.9 table, not a formula. That matters on small sizes: a real 45° ell carries a bit of extra straight, so its center-to-face does not follow 0.625 × NPS. A 2" 45 is 1.38", a 4" is 2.52", a 6" is 3.74".

With Pro, the Takeout Chart under Reference lists the B16.9 center-to-face or center-to-end of the 90° LR Elbow, 45° Elbow, 90° SR Elbow, 180° LR Return, Tee (run / branch) and Cap for NPS 1/2 to 24. At 90° the offset uses the 90° table (6.00" for a 4" LR ell); at 22.5°, 30° and 60° it uses the geometry of an ell cut down to that angle.

Set your usual weld gap once

In Settings, Default Weld Gap goes from 0 to 1/2" in 1/16" steps. Offset fills it in for you, and you can still change Weld Gap (per joint) for a single job. Our habit is 1/16" as the default and bumping it for a thicker wall rather than the other way round — forgetting to reset a wide gap is the one that bites you.

Common mistakes

• Subtracting one takeout instead of two. An offset has a fitting at each end.
• Using the 90° takeout for a 45° ell, or the long-radius value on short-radius fittings.
• Measuring the offset from the outside of the pipe rather than center to center.
• Expecting kerf or bevel prep in the result. The app does not add either.

Questions and answers

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