What a rolling offset is
When a pipe has to move sideways and up or down at the same time to get around an obstacle, plumbers and pipefitters use a rolling offset: two identical fittings, usually 45° elbows, joined by a diagonal piece of pipe. The calculator finds the length of that diagonal piece.
- Roll: the horizontal (side) offset between the two parallel runs
- Set (or rise): the vertical offset
- True offset: the straight-line distance between the two pipe centerlines
- Travel: the center-to-center length of the diagonal pipe
- Run: how far the diagonal advances along the direction of the main line
Formulas
True offset = √(roll² + set²)
Travel = true offset ÷ sin(fitting angle)
Run = true offset ÷ tan(fitting angle)
Multipliers by fitting angle
| Fitting | Travel multiplier | Run multiplier |
|---|---|---|
| 60° | 1.155 | 0.577 |
| 45° | 1.414 | 1.000 |
| 30° | 2.000 | 1.732 |
| 22.5° | 2.613 | 2.414 |
| 11.25° | 5.126 | 5.027 |
Example
A drain line must shift 12 in sideways and 16 in up. The true offset is √(12² + 16²) = 20 in. With 45° elbows, the travel is 20 × 1.414 = 28.28 in center to center and the run is 20 in.
Cut length
The travel runs from the center of one fitting to the center of the other. To get the length of pipe to cut, subtract the take-off of each fitting, the distance from its center to the point where the pipe stops inside it. This depends on the fitting type and size and is listed by the manufacturer. Enter it to get the cut length directly.
Tips
- Measure the roll and set between pipe centerlines, not pipe edges.
- Dry-fit before gluing PVC or soldering copper.
- For drains, keep the required slope on the horizontal runs on each side of the offset.
- The same method works for conduit and ductwork.