Shiplap calculator

New to this? Words explained.

How the shiplap count works

Shiplap boards overlap: each one has a rabbet (a step cut along the edge) that laps over the board below. So a board only covers its coverage width, sometimes called the face or exposure — always less than its full width. Divide the wall height by that coverage to get the rows, multiply by the wall width for the linear length, then divide by the board length.

rows = height ÷ coverage width (round up)
linear length = rows × wall width − openings ÷ coverage width
boards = linear length × (1 + waste) ÷ board length (round up)

Example: an 8 ft wall covered with boards showing 5-1/8 in each needs 96 ÷ 5.125 = 18.7, so 19 rows. On a 12 ft wide wall that is 19 × 12 = 228 linear feet; with 10% waste, 251 ft, or 32 boards 8 ft long.

Nominal size, actual width and coverage are three different numbers

• Nominal is the name on the rack — “1×6 shiplap”. Like all lumber, the board is smaller than its name (a 1×6 is about 5-1/2 in wide and 3/4 in thick when dressed; see the actual lumber size calculator).
• Actual width is the board you can measure with a tape.
• Coverage width is what’s left after the overlap — often around 5-1/8 in for a 1×6 and 6-7/8 in for a 1×8, but profiles differ, so use the figure on the label or measure a sample: nest two boards together and measure from the bottom of one to the bottom of the next.

Using the full width instead of the coverage is the usual reason people run short: on a tall wall it undercounts by a row or two per wall.

Plan the rows before you nail

• Snap a level line for the first row rather than trusting the floor or ceiling; old rooms are rarely level. A chalk line and a level make this a two-minute job.
• The calculator shows what the last row rips to. If it is a thin sliver, split the difference between the top and bottom rows so neither looks like an afterthought.
• Stagger the butt joints between rows by at least a stud bay, and land every joint on a stud if you are nailing straight to the framing (or use adhesive on drywall per the product’s instructions).
• For “nickel gap” boards, the small reveal between rows is already built into the coverage width — don’t add it again.
• Let solid-wood boards sit in the room for a few days first; they move with humidity just like any other board (the wood shrinkage calculator shows how much).

Waste and openings

Openings are subtracted by area, which is close enough for doors and windows: you still cut around them, but you don’t need boards for the hole. Ten percent waste covers end cuts on a plain wall; go to 15% with several windows, outlets to notch around, or a sloped ceiling where every row ends in an angled cut. Longer boards waste less at the ends but are harder to get home — the cut list calculator helps if your wall is an awkward length.