The formulas
- True offset = square root of (rise squared + roll squared)
- Travel = true offset / sin(angle)
- Run = true offset / tan(angle)
- Multiplier = 1 / sin(angle)
- Shrink = true offset x tan(angle / 2)
- Cut length = travel - 2 x take-off
- Fitting roll angle from level = arctangent(rise / roll)
Where they come from
Look at the offset from the end of the pipe. The rise and the roll are the two legs of a right triangle. The true offset is the long side.
Now look at the true offset from the side. It is one leg of a second right triangle. The travel is the long side, and the fitting angle sits between the travel and the run. That gives the sine and the tangent in the formulas.
Three worked examples
| Rise | Roll | True offset | Travel | Run | Roll fittings to |
|---|---|---|---|---|---|
| 6" | 8" | 10.00" | 14 1/8" (14.14) | 10" | 36.9° from level |
| 12" | 9" | 15.00" | 21 3/16" (21.21) | 15" | 53.1° from level |
| 24" | 7" | 25.00" | 35 3/8" (35.36) | 25" | 73.7° from level |
The fitting roll angle
When you set the two fittings, they must lie in the plane of the true offset. For a rise of 12 and a roll of 9, that plane tilts 53.1 degrees from level, or 36.9 degrees from plumb. The tool shows both numbers.
Questions
What is the rolling offset formula?
The true offset is the square root of rise squared plus roll squared. The travel is the true offset divided by the sine of the fitting angle.
Is the rolling offset formula the same for pipe and conduit?
Yes. Pipe adds a take-off for each fitting. Conduit adds shrink for the bends.