What this calculator does
A rolling offset moves a pipe in two directions at once, so the true offset is the diagonal of the rise and the set. A 12 inch rise with a 9 inch set gives a true offset of 15 inches, which is the 3-4-5 triangle in disguise.
The fitting angle then decides the cut length. At 45 degrees the travel is 21.21 inches and the run is also 15; at 22.5 degrees the travel stretches to 39.20 inches and the run to 36.21. A shallower fitting needs much more room along the pipe, which is often the constraint that decides which fitting to use.
The formula
The true offset is the hypotenuse of the rise and set by Pythagoras. The travel, which is the centre-to-centre cut length between the two fittings, is the true offset divided by the sine of the fitting angle. The run, how far the pipe advances along its original direction, is the true offset divided by the tangent.
| Term | Meaning |
|---|---|
| Rise | The vertical component of the offset. |
| Set | The horizontal component, perpendicular to the pipe run. |
| True offset | The diagonal combining rise and set: the actual distance the pipe shifts. |
| Travel | The centre-to-centre length between fittings, which is what you cut. |
The inputs explained
| Field | What to enter |
|---|---|
| Rise (vertical offset) (in) | Vertical offset in inches. |
| Set (horizontal offset) (in) | Horizontal offset, perpendicular to the original pipe direction. |
| Fitting angle | Fitting angle. 45 degrees is most common; shallower angles need more run but restrict flow less. |
When to use it
Routing pipe around an obstacle
Where the pipe must shift both sideways and vertically, the rolling offset is the standard solution.
Choosing a fitting angle
The run figure shows whether there is room along the pipe for a shallow fitting.
Cutting to length
The travel is the dimension to cut, before deducting fitting take-off.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the fitting angle change the cut?
The same offset made with a range of fitting angles.
| Fitting angle | Travel (cut length between fittings) | Run | True offset |
|---|---|---|---|
| 22.5° | 39.20 in | 36.21 in | 15.00 in |
| 30° | 30.00 in | 25.98 in | 15.00 in |
| 45° | 21.21 in | 15.00 in | 15.00 in |
| 60° | 17.32 in | 8.66 in | 15.00 in |
| 90° | 15.00 in | 0.00 in | 15.00 in |
Questions
What is a rolling offset?
A pipe offset that shifts in two directions at once, both vertically and horizontally, rather than in a single plane. It is common where pipe has to dodge a beam or another service, and it is calculated by first combining the two shifts into one diagonal.
What is the difference between travel and run?
Travel is the length of pipe between the two fittings, which is what you cut. Run is how far the pipe advances along its original direction while making the offset, which is what determines whether it fits the available space.
Do I deduct for the fittings?
Yes. The travel is centre to centre between fitting centres, so the actual pipe cut is shorter by the take-off of each fitting plus the socket depth. Take-off dimensions come from the fitting manufacturer and vary by material and size.
Which fitting angle should I use?
45 degrees is the usual default: it balances the run needed against the flow resistance. Shallower angles such as 22.5 restrict flow less, which matters on drainage, but need considerably more run. 90 degree fittings need no run but disturb flow most.
For drainage falls, see the pipe fall calculator. For pipe volumes, see the pipe volume calculator.