Rolling Offset Calculator
A rolling offset occurs when a pipe must clear an obstacle by changing direction in both the horizontal (roll) and vertical (rise) planes at the same time. Enter the rise, roll, fitting angle, and fitting takeoff to get the true offset, travel length, run, and field-ready cut length. Switch between imperial (inches) and metric (millimetres) with one click.
What is a rolling offset?
A rolling offset, also called a combination offset or compound offset, is a pipe routing move that changes direction in two planes simultaneously: the pipe rises or drops (vertical offset) while also shifting sideways (horizontal offset). This is different from a simple offset, which moves in only one plane. Rolling offsets appear constantly in commercial plumbing, HVAC ductwork, and industrial piping whenever a run of pipe must clear a beam, another pipe, or a structural member that sits diagonally out of both the horizontal and vertical paths.
How the rolling offset formula works
The calculation rests on two applications of the Pythagorean theorem. First, the rise and roll are treated as the two legs of a right triangle whose hypotenuse is the "true offset" (also called the diagonal offset). True offset = sqrt(rise squared + roll squared). Second, the true offset and the fitting angle are used to find the travel length, which is the center-to-center distance of the spool piece between the two elbows. Travel = true offset / sin(angle), or equivalently, travel = true offset times the fitting multiplier. For a 45-degree elbow the multiplier is 1.4142 (square root of 2), the same constant used in simple 45-degree offsets. The run, which is how far the pipe advances in the main flow direction, is found from run = true offset / tan(angle). Finally, the cut length for the spool piece is travel minus twice the fitting takeoff, because each elbow consumes one takeoff length at each end.
Fitting takeoff and cut length
The fitting takeoff is the center-to-face dimension of each elbow. When two elbows are used to make the rolling offset, you must subtract one takeoff for each fitting from the travel to get the field cut length. The cut length is what the pipefitter actually marks and cuts on the pipe: cut length = travel - (2 times takeoff). Takeoff values vary by pipe diameter and material. For threaded steel pipe, a 1-inch fitting typically takes off about 0.83 inches; for copper and press fittings the values are slightly different. Consult the fitting manufacturer table or the pipe-trade reference for your specific material. If you have not accounted for any end make-up (thread engagement or insertion depth) you may need to subtract that separately.
Common applications in plumbing and HVAC
Rolling offsets are used wherever a pipe must route around a structural member, change elevation while also moving sideways, or connect two runs whose inlets are not aligned in a single plane. Typical situations include: transitioning a horizontal main around a steel beam that sits both above and to the side of the pipe path; connecting a branch at a non-orthogonal angle from a riser; routing plumbing between floors where the slab penetrations are offset in both directions. In sprinkler system layouts, rolling offsets in branch lines are especially common because the layout follows the geometry of the ceiling grid rather than simple horizontal or vertical moves.
Travel multipliers by fitting angle
| Fitting angle | Travel multiplier (1/sin) | Notes |
|---|---|---|
| 11.25 degrees | 5.1258 | Shallow sweep, long run |
| 22.5 degrees | 2.6131 | Common in DWV systems |
| 30 degrees | 2.0000 | Moderate sweep |
| 45 degrees | 1.4142 | Most common commercial fitting |
| 60 degrees | 1.1547 | Steep sweep, short run |
| 90 degrees | 1.0000 | Travel equals true offset |
Multiply the true offset by the multiplier to find the center-to-center travel for common elbow angles. The 45-degree fitting (multiplier 1.4142) is the most widely used in commercial plumbing.
Frequently asked questions
What is the rolling offset formula?
True offset = sqrt(rise squared + roll squared). Travel = true offset divided by sin(fitting angle), which is the same as true offset times the fitting multiplier. Run = true offset divided by tan(fitting angle). Cut length = travel minus (2 times fitting takeoff). For a 45-degree elbow the multiplier is 1.4142, so travel equals the true offset times 1.4142.
What fitting multiplier do I use for 45-degree elbows?
The multiplier for a 45-degree fitting is 1.4142, the same as the square root of 2. Multiply the true offset by 1.4142 to get the center-to-center travel. For a 22.5-degree fitting use 2.6131, for 60-degree use 1.1547, and for a 90-degree fitting the multiplier is 1.0000 (travel equals true offset).
What is the difference between a simple offset and a rolling offset?
A simple offset changes direction in one plane only: either horizontally (roll only) or vertically (rise only). A rolling offset changes direction in both planes at the same time. The calculation is similar but requires one extra step: combine the rise and roll into a true offset using the Pythagorean theorem, then treat that diagonal as if it were the offset in a standard single-plane calculation.
How do I find the cut length after calculating travel?
Subtract twice the fitting takeoff from the travel. The takeoff is the center-to-face measurement of each elbow. Because there is one elbow at each end of the spool piece, you subtract two takeoffs. Cut length = travel - (2 times takeoff). If the result is negative, your fittings are too large for the available travel, and you need to use a different routing or smaller-dimension fittings.
Can this calculator handle metric dimensions?
Yes. Switch the unit selector to Metric and enter rise and roll in millimetres. The true offset, travel, run, and cut length are all returned in millimetres. The fitting multipliers and formulas are unit-independent: they apply equally to inches, millimetres, feet, or any consistent length unit.
What if I only have the run length, rise, and roll but not the travel?
If all three spatial dimensions are known, travel = sqrt(run squared + rise squared + roll squared). This is the three-dimensional diagonal of the rectangular box formed by the run, rise, and roll. Our calculator works from rise and roll plus a fitting angle, which is the most common field scenario where the fitting angle is known but the run is what you want to find.