Shiplap Calculator
Count shiplap boards for a wall or ceiling from the board's exposed face width — rows, boards per row, the total to buy and packages.
How shiplap coverage really works
Shiplap is sold by the board but covers by the face: each horizontal row climbs exactly one exposed face width up the wall — 5.5 in for a 1 × 6 — because the rabbeted edge of the next board laps over the last. Count the rows your wall height needs, multiply by the boards each row takes at your board length, and the wall is counted.
Rows = wall height ÷ exposed face width, rounded up · Boards = rows × boards per row (wall length ÷ board length, rounded up)
Example: a 12 ft long, 8 ft high wall in 1 × 6 shiplap (5.5 in face) with 8 ft boards: 96 ÷ 5.5 = 18 rows, 2 boards per row, 36 boards exactly — 40 to buy with the 10% allowance.
| Board | Exposed face | Rows per 8 ft wall | Coverage per 8 ft board |
|---|
| 1 × 4 | 3.5 in | 28 | ≈ 2.3 sq ft |
| 1 × 6 | 5.5 in | 18 | ≈ 3.7 sq ft |
| 1 × 8 | 7.25 in | 14 | ≈ 4.8 sq ft |
Longer boards mean fewer butt joints — on a 12 ft wall, 12 ft boards take the seams out entirely, and joints that remain should be staggered row to row and landed where they draw the least eye. Leave a 1/4 in expansion gap at the floor, ceiling and side walls; the baseboard and trim hide it, and wood that cannot move will buckle a row off the wall in a humid summer instead. For a ceiling, the same count applies with the wall height read as the ceiling's short dimension.
Frequently asked questions
What is the difference between shiplap and tongue and groove?
Shiplap boards overlap on a rabbet (an L-shaped edge) and traditionally show a small shadow gap — the nickel gap, sized by a coin's thickness. Tongue and groove boards interlock and sit flush. Coverage maths is the same: exposed face width rules both.
Can shiplap run vertically?
Yes — swap the thinking: the rows become columns. Enter the wall's width as the height in the form (it sets the row count) and the height as the length, or simply run the numbers twice; the board count is identical by symmetry.
Should I prime the back of the boards?
For exterior or bathroom work, yes — sealing all faces slows moisture cycling and cupping. Indoors in dry rooms it is optional; priming the tongues and gaps of pre-finished boards is not needed.
How much does the nickel gap change coverage?
Very little — the gap is part of the shadow line the face width already assumes. The calculator counts on the full exposed face (5.5 in for 1 × 6), which matches the coverage printed on most shiplap packaging.
CalcCoverage