Board and batten spacing formula on a wall elevation — a batten in every corner

With a batten at each end, n battens make (n − 1) equal gaps. Solve the count from your target, then read the true spacing: gap = (L − n·w) ÷ (n − 1).

What this diagram shows

A wall elevation showing evenly spaced vertical battens with a batten at each end (in every corner) under a horizontal top rail. Because there is a batten at each end, five battens create four equal gaps — for n battens there are (n − 1) gaps. The wall length is labeled L, one batten's actual width is labeled w (for example a 1×4 is 3½ inches), and one open space is labeled gap s. The formula is gap = (L − n × w) ÷ (n − 1). To solve the count from a target spacing, n_gaps = round((L − w) ÷ (target + w)) and n battens = n_gaps + 1. A batten in each corner keeps any partial gap from being orphaned against an adjacent wall.