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How to cut down a 90 degree elbow: the elbow cutting formula

An ell cut down to angle θ has a center-to-face of R × tan(θ ÷ 2), where R is the elbow radius (1.5 × NPS for long radius, 1.0 × NPS for short radius). A 4" LR ell cut to 30° has R = 6" and a new center-to-face of 1-5/8". FitterMaster's Odd-Angle Cut gives you that, the radius and the set-back.

2 min read · Updated 2026-09-15

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When do you cut an elbow down to an odd angle?

Lines do not always turn at 45° or 90°. When the iso calls for 30°, 22.5° or 60°, or an odd angle to line up with existing pipe, the usual fix is to cut a stock 90° butt-weld ell down to the angle you need.

The number you need afterwards is the new center-to-face, so you can subtract it from the travel like any other takeout. It is easy to forget until the spool comes up short, which is why this screen is worth opening first on an odd-angle job.

What is the elbow cutting formula?

  • Elbow radius R = 1.5 × NPS for long radius, 1.0 × NPS for short radius.
  • New center-to-face = R × tan(θ ÷ 2).
  • Set-back from the 90° face = R − new center-to-face. This is how much shorter the cut ell is than the stock 90° center-to-face.

How do you cut an elbow in FitterMaster?

  1. Open Odd-Angle Cut under Calculate (Pro).
  2. Under Elbow, pick the Size and the Series, LR or SR.
  3. Under Cut a 90° ell down to θ, set Cut Angle θ with the slider (5° to 89°, in 0.25° steps) or tap a quick angle: 11.25°, 15°, 22.5°, 30°, 33.75°, 45°, 60° or 67.5°.
  4. Read New Center-to-Face, Elbow Radius R and Set-back from 90°.
  5. Tap Copy or Share to send the result.

Worked examples

FitterMaster Odd-Angle Cut, long-radius ells
ElbowθElbow Radius RNew Center-to-FaceSet-back from 90°
4" LR30°6"1-5/8" (1.6077")4-3/8"
6" LR22.5°9"1-13/16" (1.7902")7-3/16"
2" LR60°3"1-3/4" (1.7321")1-1/4"
4" LR45°6"2-1/2" (2.4853")3-1/2"

In millimetre mode the 4" LR at 30° reads 41.0 mm, 152.5 mm and 111.5 mm.

Cutting a 90 to a 45: buy the 45 if you can

By geometry, a 4" LR ell cut to 45° has a center-to-face of 2.4853". A stocked B16.9 45° ell of the same size is 2.52", because factory 45s carry a little extra straight. Both round to 2-1/2" on a tape, but on smaller sizes and long runs the difference adds up. When a real 45 is available, use it and use its table takeout.

Tips before you cut

  • Mark the cut line square to the elbow's own bend, on the plane that passes through the center of the bend radius.
  • The app gives the center-to-face math. It does not show where to mark along the throat and heel, and this screen has no diagram, so use your usual layout method (wrap-around and protractor, or a marking tool).
  • Remember the bevel: a cut ell still needs its weld prep, and the app does not allow for it.
  • Short-radius ells are made from NPS 1 up; pick SR only if that is what you have.

Questions and answers

How do you calculate the takeout of a cut elbow?
Multiply the elbow radius by the tangent of half the angle. For a 4" LR ell (R = 6") at 30°: 6 × tan 15° = 1.6077", about 1-5/8".
Which angles can I cut to in the app?
Any angle from 5° to 89° in 0.25° steps, set with the slider or the quick buttons.
Is Odd-Angle Cut free?
No, it is part of the one-time Pro upgrade.
Does the offset calculator use the same formula?
Yes, for 5.625°, 11.25°, 22.5°, 30° and 60°, and for 45° short radius. At 90° and 45° long radius it uses the B16.9 table.

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