Decide what happens at the edges
With battens flush at both ends, N battens create N − 1 clear spaces. With equal spaces at both ends as well as between battens, the same N battens create N + 1 spaces. Those two layouts have different answers even when the wall and boards are identical.
A worked 122-inch wall
Seven battens measuring 2 inches wide cover 14 inches. With flush edge battens, the remaining 108 inches are divided into six gaps of 18 inches. Add one batten width to get 20-inch centre spacing. The left edges sit at 0, 20, 40, 60, 80, 100 and 120 inches.
Mark from one reference edge
Use the position list to mark each left edge from the same starting point. Repeatedly adding a rounded gap can accumulate error. Decimal values are shown to two places; keep the final edge check on site and do not assume the wall is perfectly straight or square.
Before buying or fixing boards
The height total is only the combined length of vertical battens. It excludes rails, trim, saw kerfs, stock-length cutting waste and spare material. Check outlets, switches, skirting and obstructions separately. This is a decorative spacing aid, not structural or code-compliance advice.
Questions, answered
Is the result a clear gap or centre spacing?
Both are labelled. The clear gap is the empty distance between adjacent boards. Centre spacing adds the actual batten width.
Can I enter millimetres?
Yes. Choose millimetres to convert the existing dimensional fields. All measurements within one calculation use the same unit.