Courses and exposure
Clapboard (lap) siding is installed in overlapping horizontal courses. The exposure is the visible portion of each board. A board that is 6 inches wide with a 1.5-inch overlap has a 4.5-inch exposure. The number of courses on a wall equals the wall height divided by the exposure, rounded up.
Total board count is the number of courses multiplied by the wall width divided by the board length, accounting for end joints. Standard clapboard lengths are 8, 10, 12, or 16 feet.
Courses = ceil(Wall height / Exposure)
Exposure is the visible width per course after overlap.