What Is Shrink in Conduit Bending?
By Andrew ·
Quick answer
Shrink is how much shorter a run of conduit gets when you bend an offset or a saddle into it, because part of it travels on a slant. Shrink = offset height × tan(angle ÷ 2): 0.268" per inch at 30°, about 1/4" by the trade’s rule. A 6" offset at 30° shrinks 1 5/8". Add it when you measure toward an obstruction.
Why conduit shrinks
An offset sends the conduit along the slanted side of a triangle for a short way. That slant is longer than the distance it covers along the run, so conduit past the offset ends up closer to where it started than it would on a straight stick. The difference is the shrink.
shrink = offset height × tan(angle ÷ 2)Nothing before the first bend moves. Everything past the second bend comes back toward the end you measured from by the shrink. A saddle shrinks the same way, once for each slope.
Shrink per inch, exact and by the rule
| Angle | Exact per inch | Rule per inch | 6" offset, exact | 6" offset, by the rule |
|---|---|---|---|---|
| 10° | 0.087" | 1/16" | 1/2" | 3/8" |
| 22.5° | 0.199" | 3/16" | 1 3/16" | 1 1/8" |
| 30° | 0.268" | 1/4" | 1 5/8" | 1 1/2" |
| 45° | 0.414" | 3/8" | 2 1/2" | 2 1/4" |
| 60° | 0.577" | 1/2" | 3 7/16" | 3" |
The rules are easy to carry in your head and close enough on small offsets. They all run a little short, though, and the gap grows with the height and the angle: on a 6" offset it is about 1/8" at 30°, 1/4" at 45° and 7/16" at 60°. On a run with several offsets, those add up.
When to add shrink to a mark
Measured from the end of the conduit, the bend nearer the end lands where you mark it. The bend farther from the end lands one shrink closer to the end than its mark. That decides whether the shrink goes into your marks.
- Toward an obstruction. The offset has to be finished by a point you measured to, such as the face of a beam or the edge of a box. Add the shrink to that measurement and mark the bend nearer the obstruction there. The other mark goes height × multiplier back toward the end.
- Away from an obstruction. The offset is placed by the bend nearer the end, and nothing past it has a fixed length yet. The marks need no shrink, but the conduit past the offset comes up short by it, so measure the rest of the run after bending, or add the shrink to the cut length.
- Offset height
- 4"
- Angle
- 30°
- End to where the offset must be finished
- 40"
- Shrink
- 1 1/16"
- Mark for the bend nearer the obstruction
- 41 1/16"
- Mark for the bend nearer the end
- 33 1/16" (8" back)
Shrink in saddles
| Obstruction height | 45° center, 22.5° outer bends | 60° center, 30° outer bends |
|---|---|---|
| 1" | 3/8" | 9/16" |
| 2" | 13/16" | 1 1/16" |
| 3" | 1 3/16" | 1 5/8" |
| 4" | 1 9/16" | 2 1/8" |
When you measure from the end to the middle of the obstruction, move the center mark out by half the saddle’s shrink: the first slope uses that much up before the conduit reaches the pipe. A 4-point saddle is two offsets, so it shrinks twice as much as one offset of the same height.
Real benders shrink a little less
The formula treats each bend as a sharp corner. A real shoe curves round the corner, which saves a little conduit at each bend, so the true shrink is slightly smaller. The difference grows with the shoe’s radius and the angle. BendMate works out marks and shrink from your bender’s centerline radius and deduct, so the numbers match the bender in your hands.
Get the marks for your bender
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Common questions
How do you calculate shrink on an offset?
Multiply the offset height by the shrink per inch for the angle, which is tan(angle ÷ 2). At 30° it is 0.268", so a 4" offset shrinks 1 1/16".
What is the shrink for a 45 degree offset?
0.414" per inch of offset height, which the trade rounds to 3/8". A 6" offset at 45° shrinks 2 1/2".
Do you add or subtract shrink?
Add it to your measurement when the offset has to be finished at a point measured from the end of the conduit. When the bend nearer the end sets the offset’s place, the marks stay as they are, but the run past the offset ends up short by the shrink.
Does a 90 degree bend have shrink?
Not in this sense. Shrink belongs to offsets and saddles. For a 90 you use the take-up to place the mark and the gain to work out the cut length.