Conduit shrink calculator

Shrink is what an offset takes out of the run, and it is the number people forget. Enter the rise of your offset and the bend angle; the calculator gives the shrink to add to your measurement, so the pipe still reaches the far fitting after both bends pull the end back.

Worked example

You are running to a box 40 inches away and need a 6 inch offset at 30° along the way. The shrink for 30° is about 0.268 inch per inch of rise, so 6 × 0.268 ≈ 1 5/8 inches. If you measured the offset to start at 20 inches, mark it at 21 5/8 instead. Skip that and the run finishes about an inch and a half short of the box.

The formula

Shrink per inch of rise is one over the sine of the angle, less one over the tangent:

shrink = rise × (1 ÷ sin θ − 1 ÷ tan θ)

The pipe between the marks is the hypotenuse of a right triangle, length rise ÷ sin θ. The horizontal ground it covers is only rise ÷ tan θ. The difference between the sloped pipe and the flat ground it replaces is the run you lose — that is the shrink. It falls out of the same triangle as the multiplier, so there is no separate table to trust.

Shrink per inch, exact vs. the field rule

The exact figure is per inch of rise; multiply by your rise. The field rules are the rounded per-inch amounts a lot of benders keep in their head. They are close on a small offset and drift on a large one.

Questions

What is shrink in conduit bending?

Shrink is the length an offset pulls out of the run. Bending the pipe up and back over pulls the far end toward the bends, so the finished run is shorter than the flat length you measured. Shrink is how much shorter, and you add it to your measurement before the first mark so the end still lands where you want it.

Do I add shrink or subtract it?

Add it. Shrink is a loss, so you compensate by moving your first mark further along the pipe by the shrink amount. Measure to where the offset should start, add the shrink, and make the first mark there. Subtracting it sends the far end short by twice the figure.

Which angle shrinks the most?

The steeper the bend, the more it shrinks per inch of rise. A 60° offset shrinks about 0.58 inch per inch of rise; a 10° offset only about 0.09. That is one reason a shallow angle is easier to place in a long run — it pulls the end back far less.

Does shrink depend on the pipe size?

No. Shrink per inch of rise comes only from the bend angle, so it is the same for half-inch EMT and two-inch rigid. What changes with pipe size is take-up and gain on a 90, not the offset shrink.

Related tools

• Conduit offset calculator — the marks and the shrink together.
• Conduit bending multipliers — the multiplier beside the shrink for each angle.
• Rolled offset calculator — shrink against the true offset of a tilted jog.