Start an arch drawing with two springing points and the required height above their connecting line. For a circular segmental arch with level springings, these dimensions determine the circle. Other arch shapes need different constructions.
Measure span and rise
Span is the straight distance between the springing points. Rise is measured perpendicular to that line up to the crown of the inner curve. Do not confuse it with the thickness of the arch ring or the brick depth through the wall.
Calculate a segmental arch radius
Multiply the span by itself. Divide by eight times the rise, then add half the rise. For a 1600 mm span and a 400 mm rise: 1600² ÷ 3200 + 200 = 1000 mm. This formula applies to a circular arc, not every arch shape.
1 600² ÷ (8 × 400) + 400 ÷ 2 = 1 000 mm
Locate the centre and draw the intrados
Draw the opening centreline. In this example the circle centre lies 1000 − 400 = 600 mm below the springing line. Measure 1000 mm from this point to both springings and the crown. Use that radius to draw the intrados. For a constant arch ring, draw the outer curve with the radius plus the ring thickness.
Divide along the curve, not the clear span
The curved line is longer than the straight opening. Brick and joint dimensions belong to the chosen reference line. Basket-handle and Tudor arches use multiple centres; an ellipse does not have one fixed circular radius. Use each shape’s own calculator and setting-out instructions.