Rolling Offset Calculator
By Andrew ·
Quick answer
A rolling offset moves conduit up and sideways at once with a single offset bent on the diagonal. True offset = √(rise² + roll²), and the bends are rolled atan(roll ÷ rise) out of plumb. Then mark it as an ordinary offset of the true height. With 5" rise and 12" roll the true offset is 13", rolled 67.38°.
Calculator
- Results
- True offset
- 10"
- Roll angle
- 53.13°
- Distance between bends
- 20"
- Shrink
- 2 11/16" (2.68")
Worked out the textbook way, from the bend angle alone. BendMate also uses your bender’s radius, deduct and shoe.
Why one diagonal offset beats two
When a box sits up and to the side of the run, you could bend one offset for the rise and another for the roll. That wastes conduit and adds bends to pull through. A rolling offset reaches the same spot with one pair of bends, along the diagonal between where the pipe is and where it has to go.
true offset = √(rise² + roll²)roll angle = atan(roll ÷ rise)Worked example: 5" rise, 12" roll
- Rise
- 5"
- Roll
- 12"
- Angle
- 30°
- True offset
- √(5² + 12²) = 13"
- Roll angle
- 67.38°
- Marks apart
- 13" × 2.000 = 26"
- Shrink
- 13" × 0.268 = about 3 1/2"
- Bend an ordinary 30° offset with the marks above, turning the conduit half a turn between bends.
- Lay the bent conduit in place and turn it on its own axis until the bends lean 67.38° from plumb, toward the side it has to move.
- Check that the far leg lands at the right height and the right distance over before you fasten it.
Numbers that work out clean
Rise-and-roll pairs from a right triangle with whole sides make the diagonal easy to check in your head.
| Rise | Roll | True offset | Roll angle |
|---|---|---|---|
| 3" | 4" | 5" | 53.13° |
| 6" | 8" | 10" | 53.13° |
| 5" | 12" | 13" | 67.38° |
| 8" | 15" | 17" | 61.93° |
Where rolling offsets go wrong
- Adding rise and roll instead of using the diagonal. Here that would give 17" for a true offset of 13", so the marks end up far too wide.
- Measuring rise to the bottom of the conduit and roll to its center.
- Rolling the bends to the wrong side, so the pipe moves away from the box.
- Applying shrink for the rise alone. The shrink belongs to the true offset.
Your bender and the true offset
Once the true offset is known, a real shoe treats it like any other offset, with slightly closer marks than the chart. In BendMate the same math accounts for your bender’s radius and deduct.
Get the marks for your bender
BendMate works this out from your bender’s radius and deduct, flags a bend that won’t work, and shows you every mark.
Common questions
How do you calculate a rolling offset?
Work out the true offset with √(rise² + roll²), then mark and bend an ordinary offset of that height. Finally roll the conduit so the bends lean atan(roll ÷ rise) from plumb.
What is the difference between rise and roll?
Rise is how far the conduit has to move up or down, and roll is how far it has to move sideways. Both are measured center to center.
How much shrink does a rolling offset have?
It is the shrink of an ordinary offset of the true height. A 13" true offset at 30° shrinks about 3 1/2".
Which angle should I use for a rolling offset?
30° is the usual choice. Use 45° when there is little room along the run, and a smaller angle when the true offset is small and the bends would crowd each other.