How to measure a ductwork offset on site
A reliable offset begins with matching centreline measurements. The calculator can do the arithmetic, but it cannot correct measurements taken from different faces or an assumed bend size.
The four values you need
- Rise or up: the vertical distance between the two duct centrelines.
- Offset or over: the sideways distance between the same two centrelines.
- Bend angle: normally the marked angle of the two equal bends being used.
- Bend take-off: the distance removed by each finished fitting, taken from the correct manufacturer table or a controlled measurement.
Straight offset versus rolled offset
A straight offset moves in one plane. A rolled offset moves vertically and sideways at the same time. The two entered movements are combined into one true resultant before the bend angle is applied.
The travel is the distance along the sloping centreline between the bend intersections. The straight cut is shorter because the two bends occupy part of that travel.
Worked site example
Assume a rise of 300 mm, a sideways offset of 250 mm, two 45° bends and a published take-off of 130 mm per bend.
| Stage | Calculation | Result |
|---|---|---|
| True movement | √(300² + 250²) | 390.5 mm |
| Travel at 45° | 390.5 ÷ sin 45° | 552.3 mm |
| Two bend take-offs | 2 × 130 | 260 mm |
| Straight between bends | 552.3 − 260 | 292.3 mm |
That is the cut length before any separate allowance required by the actual jointing or fabrication method. VentTools shows the working so the result can be checked rather than accepted as an unexplained number.
Calculate this offsetA practical measuring sequence
- Confirm the route and final duct centreline on the drawing and on site.
- Mark or establish the starting and finishing centrelines.
- Measure vertical rise and sideways movement independently.
- Confirm the two physical bends are the same diameter, angle and family.
- Check their published or measured take-off.
- Calculate the straight, then check the assembled offset can physically rotate into position.
- Allow for insulation, flanges, seams, access and supports before cutting.
Common mistakes
- Using face-to-face measurements when the calculation expects centreline-to-centreline movement.
- Using only the rise on a rolled offset and forgetting the sideways movement.
- Subtracting a theoretical radius instead of the finished bend take-off.
- Assuming every manufacturer's pressed bend has the same take-off.
- Cutting a very short centre piece without checking whether opposing spigots or swages will meet.
- Forgetting that an offset that works mathematically may still clash while being assembled or lifted.
Where the calculator stops
The offset calculator is a cutting and setting-out aid. It does not approve a change to the designed route, support arrangement, duct size, insulation clearance or fire strategy. Check the drawing and obtain approval where the proposed installation differs.