Side sections left & right pair
Middle section odd count · key brick
Joint rule used
Side sections: joints = bricks.
Middle section: joints = bricks + 1.
Therefore total joints = total bricks + 1.
Construction basis
The large construction arc is a quadrant struck from the centre of the span to the springing point.
The small upper construction arc is struck from the top of rise with a radius equal to the distance from the rise point to where the large arc meets the centre line.
The remaining chord from springing point to that upper-arc intersection is bisected at 90°.
Selection logic
The gauge and joint values can be adjusted to suit imperial work.
For each section, the calculator chooses the arrangement that gets the extrados brick width as close as possible to the selected maximum without exceeding it.
The middle section is restricted to an odd number of voussoirs so there is always a centred key brick.