Rolling Offset Calculator

Rolling Offset Calculator finds the travel and the pipe cut length for a rolling offset from roll, rise and fitting angle. It also gives the fitting roll angle and the run it uses.

Rolling Offset Calculator Inputs and Cut Length

The Rolling Offset Calculator needs three measurements to start. Roll is the sideways distance between the two pipe lines. Rise is the vertical distance between them. The fitting angle is the elbow you plan to use at each end, usually 45°.

Measure roll and rise between pipe centerlines, not between pipe faces. When both pipes are the same size, the distance from the top of one to the top of the other is the same as center to center. With different sizes, add or subtract half of each outside diameter.

The headline result is the pipe to cut, not only the travel. With a 12 in roll, a 16 in rise and 45° weld elbows on NPS 2 pipe, the travel is 28-5/16 in and the cut length is 25-5/16 in. The cards add the true offset, the fitting roll angle, the run the offset uses and a check against the room you have.

Rolling Offset Formula

A rolling offset moves the pipe sideways and up at the same time. The roll and the rise form two sides of a right triangle. The true offset is the diagonal between the two pipe lines.

$$\text{True Offset}=\sqrt{{\text{Roll}}^2+{\text{Rise}}^2}$$

The travel is the center-to-center distance between the two fittings. It is the true offset divided by the sine of the fitting angle. For a 45° fitting that is the same as multiplying by 1.414.

$$\text{Travel}=\frac{\text{True Offset}}{\sin A}$$

The run is how much length the offset uses along the direction of the pipe. It is the true offset divided by the tangent of the fitting angle. With 45° fittings the run equals the true offset.

$$\text{Run}=\frac{\text{True Offset}}{\tan A}$$

Each standard fitting has a fixed pair of multipliers. The table shows them with the travel and run for a 20 in true offset. The Rolling Offset Calculator lists the same comparison for your offset under Same Offset With Other Fittings.

45 Degree Rolling Offset Example

The default entries in the Rolling Offset Calculator make a clean worked example. Each figure below appears on a card, so the steps can be checked against the result on screen.

1. Square the roll and the rise. 12 squared is 144 and 16 squared is 256, which add to 400.
2. Take the square root. The true offset is 20 in.
3. Multiply by 1.414 for 45° fittings. The travel is 28.28 in, or 28-5/16 in.
4. Take off both elbows and both weld gaps. Two take-offs of 1-3/8 in and two gaps of 1/8 in come to 3 in.
5. Subtract. The pipe to cut is 25.28 in, or 25-5/16 in.

A simple offset works the same way with one measurement set to zero. Enter a 16 in rise with no roll and the travel for 45° fittings is 22-5/8 in. The fittings are not rolled in that case, and the Rolling Offset Calculator says so in the note under the cards.

From Travel to Cut Length

Travel is measured between fitting centers, so it is longer than the pipe that goes between them. Open Fittings, Joints and Room in the Rolling Offset Calculator to set the take-off, which is the center-to-end size of one fitting. The cut length removes two take-offs and two joints from the travel.

$$\text{Cut}=\text{Travel}-2\times \text{Take-Off}-2\times \text{Gap}$$

The presets are long radius butt weld elbows to ASME B16.9, as listed in maker tables such as Ferrobend's 45° elbow dimensions. Picking 90° as the fitting angle swaps the list to 90° elbows, and the take-off changes with it.

Butt weld elbows are made as 45° and 90°, so an odd-angle weld fitting is usually trimmed from a 90° elbow. Its take-off is the bend radius times the tangent of half the angle. The radius of a long radius elbow is 1.5 times the nominal size.

$$\text{Take-Off}=R\tan \left(\frac{A}{2}\right)$$

For NPS 2 the radius is 3 in. A 30° fitting cut from that elbow has a take-off of 13/16 in, and a 60° fitting has 1-3/4 in. Enter the figure as a Custom take-off. The Rolling Offset Calculator reminds you to do this when the angle has no preset.

The joint matters too. A butt weld leaves a root gap at each end, which shortens the pipe, and 1/8 in is the default. Set Joint Type to Threaded or socket and the entry becomes makeup per end. Pipe screws or slips into those fittings, so the makeup is added to the cut length. The Welding Calculator covers the weld itself, and the Thread Calculator gives thread dimensions.

Rolling the Fittings to the Right Angle

A correct cut length is not enough. Both fittings must be turned so they point at each other along the diagonal. The second card in the Rolling Offset Calculator gives that angle from level and from plumb.

$$\text{Roll Angle}=\arctan \left(\frac{\text{Rise}}{\text{Roll}}\right)$$

For a 12 in roll and a 16 in rise the fittings turn 53.1° up from level, which is 36.9° off plumb. When roll and rise are equal the angle is 45° both ways. Set the first elbow with an angle finder, tack it, then match the second elbow to it.

When the Offset Does Not Fit the Space

Shallow fittings make a long offset. The same 20 in true offset uses 48-5/16 in of run with 22.5° fittings and only 11-9/16 in with 60° fittings. Enter the free length under Room Along the Run and the last card compares it with the run.

The card also names the shallowest standard fitting that fits. With 24 in of room, 45° fittings fit with 4 in to spare. With 12 in of room they do not, and the Rolling Offset Calculator points to 60°. A shallower fitting is easier on flow, so the shallowest one that fits is usually the best choice.

Rolling Offset Calculator in Metric

Switch Units to millimeters and every length converts, along with the elbow presets. A 300 mm roll and a 400 mm rise give a 500 mm true offset and 707.1 mm of travel at 45°. With NPS 2 elbows at 35 mm each and 3.2 mm weld gaps, the cut length is 630.7 mm.

Inch results are rounded to the nearest 1/16 in, with the decimal shown under the headline. Metric results are shown to 0.1 mm. On drain lines the offset also has to keep its fall, and the Pipe Slope Calculator checks that pitch.

Rolling Offset FAQ

What is the difference between an offset and a rolling offset?

A simple offset moves the pipe in one direction only, either up or sideways. A rolling offset moves it in both at once. The simple offset uses the single measured distance as its offset. The rolling offset first needs the true offset from the Pythagorean step.

The multipliers are the same for both. A 16 in simple offset with 45° fittings has 22-5/8 in of travel. Add a 12 in roll and the true offset grows to 20 in, with 28-5/16 in of travel and fittings rolled 53.1° from level.

How do I calculate a 22.5 degree rolling offset?

Pick 22.5° as the fitting angle in the Rolling Offset Calculator, or work it by hand. Find the true offset from the roll and the rise, then multiply by 2.613 for the travel and by 2.414 for the run. For a 20 in true offset that is 52-1/4 in of travel and 48-5/16 in of run.

The take-off changes as well. A 22.5° fitting cut from an NPS 2 long radius elbow has a take-off of about 5/8 in. With two 1/8 in weld gaps, the pipe to cut is 50-3/4 in.