Shiplap Calculator
Calculate how many shiplap boards you need for a wall from its size and the board reveal width.
Returns rows, board count, and a waste allowance.
Shiplap boards interlock with a small rabbet so each board overlaps the next, which means the width you see on the finished wall, the reveal, is a little less than the full board width. Counting boards starts from that exposed face width, not the size stamped on the lumber, and getting this wrong is the most common reason people run short.
The approach is the same one used for decking or any board cladding. Decide which way the boards run, usually horizontally, then divide the wall height by the coverage width of one board to get the number of rows, rounding up. Each row has to span the wall width, so multiply the rows by the wall width for the total linear feet of board. Convert that into a number of boards by dividing by the length you plan to buy.
Always add waste. Straight walls lose around 10 percent to end cuts and the occasional bad board, and a room with many windows, doors, or angled cuts can push that to 15 percent or more, since every opening leaves offcuts that are hard to reuse.
A few practical notes. Stagger the end joints from row to row so the seams do not line up, which both looks better and holds stronger. Buy from the same batch where you can, because stain color and grain vary between production runs, and account for trim and corner boards separately, since those are cut differently from the field boards.