Rolling Offset Calculator: Formula & Fitting Angle
A rolling offset calculator finds the pipe length (travel), true offset, and fitting angle needed to route a pipe through a rise, a roll, and an advance.
Simple Rolling Offset Calculator
Calculate the rolling offset (travel) in piping systems by entering the rise (vertical offset), roll (horizontal offset), and advance (distance along the run).
How it works
The rolling offset (travel) is the length of pipe between the fittings: the 3-D diagonal of the vertical rise, the horizontal roll, and the advance along the run axis. The true offset is ā(RiseĀ² + RollĀ²), and the fitting angle is arctan(True Offset Ć· Advance).
Rolling Offset = ā(RiseĀ² + RollĀ² + AdvanceĀ²)
Documentation
What is a rolling offset calculator?
A rolling offset calculator finds the length of pipe needed to join two points that differ in height, in sideways position, and along the direction the pipe run continues. In pipefitting, this diagonal length is called the rolling offset, or travel. The calculator also gives the fitting angle, the angle each end fitting must be cut or turned to.
How to calculate a rolling offset
A rolling offset happens when a pipe has to change position in three directions at once. Three measurements describe it.
- Rise: the vertical change in height between the two connection points.
- Roll: the horizontal change in position, measured at a right angle to the rise.
- Advance: the distance the pipe covers along its original run, also called the run.
Rise and roll meet at a right angle. The straight line between them is called the true offset.
True offset formula
True offset = ā(RiseĀ² + RollĀ²)
Rolling offset (travel) formula
The true offset and the advance also meet at a right angle, so the full length of pipe needed, the rolling offset, is:
Rolling offset = ā(RiseĀ² + RollĀ² + AdvanceĀ²)
If the advance is zero, this formula gives the same result as the true offset. That matches a plain 90Ā° elbow, where the pipe changes direction without moving forward first.
Fitting angle formula
Fitting angle = arctan(True offset Ć· Advance)
This is the angle, measured from the run axis, that each fitting is set to. When advance is zero, the angle is 90Ā°. When the advance equals the true offset, the angle is 45Ā°, a common angle for rolling-offset fittings.
Worked example
Take a rise of 3 units, a roll of 4 units, and an advance of 12 units.
- True offset = ā(3Ā² + 4Ā²) = ā25 = 5 units
- Rolling offset = ā(3Ā² + 4Ā² + 12Ā²) = ā169 = 13 units
- Fitting angle = arctan(5 Ć· 12) ā 22.62Ā°
The pipefitter needs a 13-unit length of pipe, cut with fittings set at about 22.62Ā°.
How to use this calculator
- Enter the rise, the vertical distance between the two connection points.
- Enter the roll, the horizontal distance between them.
- Enter the advance, the distance along the run. Leave it blank or at zero for a simple two-plane offset.
- Read the rolling offset, true offset, and fitting angle. Each result can be copied with the copy button.
Rise, roll, and advance must use the same unit, such as inches, feet, or centimeters. The calculator does not convert between units.
Why rolling offsets matter in pipefitting
Pipes often cannot run in a straight line. A beam, a duct, or another pipe may block the path, forcing the installer to route around it in both height and width while the pipe keeps moving forward. Plumbers use rolling offset calculations to route drain and supply lines around obstacles and to connect fixtures set at different heights. HVAC technicians use the same method for ductwork and refrigerant lines. Electricians apply the identical geometry to conduit, since the calculation depends only on the shape of the path, not on what travels through it.
Frequently asked questions
What is a rolling offset in pipe fitting?
A rolling offset is a length of pipe that changes direction in two planes at once: up or down, and side to side, while also advancing along the run. It is the diagonal line connecting the start and end points of that combined movement.
What is the difference between true offset and rolling offset?
The true offset is the straight-line distance covered by the rise and roll alone, ignoring the advance. The rolling offset, also called travel, adds the advance to that distance and gives the actual length of pipe needed. The rolling offset is always equal to or greater than the true offset.
What happens if the advance is zero?
With no advance, the rolling offset equals the true offset, and the fitting angle is 90Ā°. This is the case for a standard offset made with two 90Ā° elbows and no forward travel.
Does the unit of measurement matter?
The formula itself works with any consistent unit. Rise, roll, and advance must all be entered in the same unit, such as all in inches or all in centimeters, or the result will be wrong.
Can this method be used for materials other than pipe?
Yes. The calculation is pure geometry, so it applies equally to electrical conduit, ductwork, or any other rigid material routed through a rise, a roll, and an advance.
Why is the second input called "roll" instead of "run"?
"Roll" refers to the sideways offset, at a right angle to the rise. "Run" or "advance" refers to a separate measurement: how far the pipe continues in its original direction before the offset is cut in. Keeping the two terms separate avoids confusing the sideways distance with the forward distance.