Flue Offset Calculator: Pipe Length for a 45° or 30° Bend

When the stove cannot sit directly under the chimney or the point where the flue leaves the roof, the pipe has to step sideways through a pair of bends. This calculator works out how long the angled section between the bends must be and how much height it uses. The point most people miss: the shallower the bend, the more height an offset eats. A 600mm offset uses about 600mm of height at 45 degrees, but more than a metre at 30 degrees and more than two metres at 15 degrees.

The figures are centreline lengths: the distance measured along the middle of the pipe between the two bends. Socket overlaps differ between flue systems, so convert the centreline length into an order of pipe lengths using the manufacturer's own offset table, or choose an adjustable length to take up the difference.

How the offset maths works

The angled section is the long side of a right-angled triangle. The sideways offset is the side opposite the bend angle, so the length of pipe you need is the offset divided by the sine of the angle. The height that section climbs is the offset divided by the tangent of the angle.

At 45 degrees the sine is about 0.707 and the tangent is exactly 1, which is why a 45 degree offset climbs exactly as far as it moves sideways. At 30 degrees the sine is 0.5, so the pipe has to be twice as long as the offset, and the climb is about 1.73 times the offset. At 15 degrees the pipe is close to four times the offset. That is why installers working under a low ceiling or a tight register plate nearly always reach for 45 degree bends, the sharpest the rules allow.

What Approved Document J says about bends

Approved Document J asks for flues to be straight and vertical where possible, apart from the connection to a back outlet stove. Where bends cannot be avoided, it says they should be no more than 45 degrees to the vertical. A 90 degree elbow in the rising part of the flue is not acceptable, which is why this calculator stops at 45.

For a back outlet stove, Approved Document J also allows a short horizontal section at the connection, limited to 150mm. That short stub is not an offset in the sense used here. If you need to move the flue further than that, it has to rise first and then step across at an angle.

Why fewer and steeper bends draw better

Every change of direction slows the flue gases and gives soot and tar somewhere to settle. A flue with an offset needs more height overall to pull as well as a straight one, and the bends are where a sweep's brushes have to work hardest. Where the layout allows, moving the stove a little so it sits under the flue is better than adding an offset. Where it does not, keep to one pair of 45 degree bends, and make sure there is a way to sweep through them, such as an inspection door or a removable section, before the system is closed in.

This calculator does the geometry only. A wood burner installation is notifiable work in England and Wales, so the final flue design should come from a HETAS registered installer or be approved by building control, who will also check combustible clearances around the angled section.