FabLayout

Rolling offset calculator

Enter the rise, the roll and the bend angle. You get the true offset, the distance between marks, the shrink and how far to roll the conduit off vertical.

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Enter your measurements

Export from the Truemark app

The PDF cut sheet with this drawing and your marks is made in Truemark. It works offline on site and keeps your jobs.

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Mark spacingMark spacing2020″″30°30°True offsetTrue offset1010″″ShrinkShrink2211111616″″RiseRise88″″RollRoll66″″AngleAngle36.9°36.9°
Scale drawing

True offset

10in

Exact geometry

Distance between marks
In the app
Shrink
In the app
Roll angle
In the app

Every result is in Truemark

The free web version shows the main result and the drawing. Truemark gives you every mark and measurement, and exports a cut sheet. It is free to download.

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  • End view: the true offset is the diagonal of the rise and the roll.

How to lay it out

  1. Measure the rise and the roll

    Measure how far the run has to move up (the rise) and how far it has to move sideways (the roll), both centre to centre.

  2. Mark the offset

    Mark the first bend, then measure the distance between marks for the second, as for a plain offset at the true offset depth.

  3. Roll the conduit

    Roll the conduit by the roll angle off vertical before the first bend. That is what turns the offset into a rolling offset.

  4. Bend both marks

    Bend the first mark, roll the conduit 180° and bend the second mark to the same angle, keeping both bends in one plane.

Good to know

  • End view: the true offset is the diagonal of the rise and the roll.
  • FabLayout runs the same code as Truemark; the result is exact geometry.
  • Source: Plane trigonometry; same calculation as the Truemark app

Method and formula

A rolling offset moves the conduit up and sideways at the same time. Seen from the end, the rise and the roll are at right angles, so the offset you actually bend is the diagonal between them. After that it is an ordinary offset.

  • True offset = √(rise² + roll²).
  • Distance between marks = true offset ÷ sin θ.
  • Shrink = true offset × tan(θ ÷ 2).
  • Roll angle = arctan(roll ÷ rise), measured off vertical. With no roll it is 0° and you have a plain offset.

Worked example

The page defaults: an 8″ rise and a 6″ roll with 30° bends. The true offset is √(8² + 6²) = 10″. The marks are 10 ÷ sin 30° = 20″ apart, and the shrink is 2.6795″. Roll the conduit 36.87° off vertical before you bend.

Questions

What is the true offset?

The straight-line distance between the old centre line and the new one, seen from the end of the run. It is the hypotenuse of the rise and the roll, and it is the depth you bend.

How do I set the roll angle on the pipe?

Put a level or a protractor on the conduit in the bender and turn it until it reads the roll angle off vertical, then make the first bend.

Can I use a bender chart multiplier here?

Yes. Multiply the true offset by your bender's multiplier for the angle. The calculator shows the exact value, which differs slightly from rounded chart multipliers.

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