Exposure is the design decision
Clapboard and lap siding are measured by <strong>exposure</strong> — the visible height of each board after the one above overlaps it. A 6-inch board with 1-inch overlap shows 5 inches of exposure. Wall area ÷ exposure = total linear feet of siding, which the calculator converts to squares for ordering.
Work top-down from the water table
Layout starts at the bottom board and works up: set the first course off the water table, snap a line at each exposure step, and the wall rises course by course. Cedar and fiber-cement both shrink across the grain — re-nail courses at the manufacturer's specified overlap and leave the expansion gaps they specify at the ends.