What the ductwork offset calculator gives you
This calculator works out the straight piece required between two equal-angle circular bends. Enter the vertical rise, sideways offset, bend angle and duct diameter. VentTools combines the rise and offset into the true resultant movement, calculates the travel along the offset and removes both bend take-offs to give the straight cut length.
Rolled offsets are included. The roll angle tells you how the offset sits between horizontal and vertical, while the diagram gives a quick visual check before anything is cut.
How the cut length is calculated
The true resultant offset is R = √(rise² + offset²). Travel along the selected bend angle is T = R ÷ sin(angle). The straight is then L = T − 2l, where l is the take-off of one bend.
When the selected diameter and angle match the recorded BU/BFU bend table in VentTools, the tool identifies the dimension as an exact table value. Other combinations are clearly labelled as estimates and use the entered radius factor. That distinction matters on site: an estimated bend should be checked against the fittings actually delivered.
Worked example
For a 300 mm rise, 250 mm sideways offset, two 45° bends and 315 mm duct, the recorded bend take-off is 130 mm each. The resultant is 390.5 mm, travel is 552.3 mm and the straight cut is 292.3 mm. The roll angle is 50.2°.
Practical site checks
- Measure rise and sideways offset between matching duct centrelines.
- Confirm the delivered bend family and take-off before cutting.
- Use the minimum-straight warning to avoid opposing swages or spigots meeting inside the centre piece.
- Rolled offsets need their orientation checked in three dimensions; the screen diagram is schematic.
- Allow separately for any jointing, slip, socket or fabrication practice not represented by the bend take-off.
Limitations
The result assumes two bends of the same angle and geometry. It is a cutting and setting-out aid, not a substitute for checking actual fittings. If the calculator labels a take-off as estimated, verify it against the manufacturer’s current data or measure the bend before fabrication.