Practical tools for measured work
Duct Rolled-Offset Fabrication Layout
Calculate rolled-offset travel, advance and roll angle, with a printable mitered-elbow cut template.
Calculate the layout, check the diagram, then print the result for the shop or jobsite.
Vertical offset between the two duct centerlines.
Horizontal/sideways offset - this is what makes it a rolled offset rather than a same-plane offset.
Standard fabrication angle - typically 30, 45 or 60°.
Cut two of the pattern shown (mirror-image pair) - one for each elbow. Mark the dashed start line on the actual duct at the stated angle around from your top-of-duct reference before tracing the cut.
How it works
Why a rolled offset is different. A same-plane offset moves the duct sideways or up in a single direction, and both elbows can be cut and oriented with the cut's long point facing straight up. A rolled offset moves the run in two directions at once (rise and roll), so the true offset is the diagonal between them, and - critically - the miter cut has to be rotated around the duct's own circumference to point along that diagonal. Skipping that rotation is the most common way a rolled-offset fitting comes out misaligned.
Calculation. True offset = √(rise² + roll²). Roll angle (how far to rotate the cut from the rise direction) = atan(roll ÷ rise). Travel = true offset ÷ sin(elbow angle). Advance = true offset ÷ tan(elbow angle). Each elbow's cut angle (from square) is half the chosen elbow angle, using the same tilted-cylinder unroll as the segmented elbow tool.
Worked example. With the defaults (300 mm rise, 400 mm roll - a 3-4-5 triangle), the true offset is exactly 500 mm and the roll angle is 53.1° from the rise direction. At a 45° elbow angle, travel is ≈ 707 mm and advance is exactly 500 mm.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Rise
- 300 mm
- Roll (set)
- 400 mm
- Duct/pipe diameter
- 200 mm
- Elbow angle
- 45 deg
Primary result: True offset: 500.0 mm.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This lays out the two elbow cuts only; it does not size straight duct lengths beyond travel and advance, or account for duct wall thickness, liner, or flange allowances - add those separately.
For a rectangular duct, the miter cut is a straight angled line across the duct rather than the curved profile shown here (which applies to round pipe/duct); the roll angle and cut angle numbers still apply, but you won't need the printable template.
Frequently asked questions
What if my roll is zero?
With no roll, this is a standard same-plane offset - the roll angle comes out to 0°, meaning no rotation around the duct is needed; the cut's long point faces straight along the rise direction as usual.
Which elbow angle should I choose?
45° is the most common for a good balance of travel length and flow resistance; 30° and 60° are also standard stock angles. A smaller angle gives a longer, gentler offset; a larger angle gives a shorter, tighter one.
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