Formula used
Units = ceil([project area ÷ coverage per unit] × [1 + waste ÷ 100])
Worked shiplap example
Shiplap Calculator example: 177 ft² × 1.1 = 194.7 ft² of order coverage. Dividing by 100 ft² per unit gives 1.95 units, so the purchase quantity is 2 whole units.
Turn area into a material list
Round to whole boards or packages by profile and length, then validate with a row-by-row layout.
Base coverage on exposed face width
Shiplap coverage should use the installed reveal after the rabbet or tongue overlap, not the nominal board width. Wall height, stock length, end joints, pattern staggering, corners, openings, and starter or finish boards affect the count. Translate that layout into usable square-foot coverage per package before using the whole-package result.
Layout details outside the area formula
Stock length, end joints, stagger, openings, offcuts, board defects, and whether end matching is available affect count.
Questions specific to this calculation
What usable coverage should I enter for shiplap?
Use the wall or ceiling area and the installed face coverage of each board after the rabbet or tongue overlap, not its overall board width.