Shiplap Calculator (boards for a wall by visible face width)

Understanding the shiplap calculator — concept and principle

This calculator works out how many shiplap boards you need for a wall, from the wall size, openings, board width, overlap, board length and waste allowance, and how many rows run up the wall. DIY decorators use it for feature walls, cabins and bathrooms.

Area ÷ visible face: with the default inputs the result is 31 boards (74.3 m with 10% waste).

How it is calculated

Visible face per board = full width − overlap.
Net wall area = width × height − openings.
Running length = net area ÷ visible face.
Boards to buy = running length × (1 + waste) ÷ board length, rounded up.
Rows = wall height ÷ visible face, rounded up (the top row is ripped to fit).

The principle

Shiplap boards overlap, so part of each board is hidden behind its neighbour. Only the visible face covers the wall, so coverage must be calculated with the face width, not the full width. Dividing the area by the face width gives the total running length of boards, as if laid end to end. A waste allowance covers cuts at ends, around openings and damaged boards, and staggering end joints from row to row uses up offcuts.

Worked example

Default inputs: wall 360 × 240 cm, no openings, boards 140 mm with 12 mm overlap, 240 cm long, 10% waste.

Visible face = 140 − 12 = 128 mm.
Net area = 3.6 × 2.4 = 8.64 m².
Running length = 8.64 ÷ 0.128 = 67.5 m; with waste = 74.3 m.
Boards = 74.25 ÷ 2.4 = 30.9, rounded up to 31.
Rows = 2,400 ÷ 128 = 18.75, rounded up to 19.

Each point is the calculator re-run with only “Full board width (mm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Full board width (mm)	Boards to buy
35	173
70	69
105	43
140	31
175	25
210	20
280	15

How to read the result

Buy a few extra boards for warped or damaged pieces and future repairs, from the same batch. On walls with many windows or sockets, raise the waste allowance. Plan the layout so the top or bottom row is not a very thin sliver.

Common mistakes

Using the full board width for coverage.
Forgetting to subtract large openings, or subtracting small ones that create waste anyway.
Lining up all end joints in one place.
Not acclimatising timber to the room before fixing.

Keep in mind: Fix boards to studs or battens as the maker recommends, and check for hidden pipes and cables before nailing or screwing into walls.

Frequently asked questions

How many shiplap boards for a 3.6 × 2.4 m wall?

About 31 boards of 2.4 m with 128 mm visible face and 10% waste.

Why use the visible face width?

The overlap is hidden behind the next board and does not cover the wall.

How much waste should I allow?

10% is common; more for walls with many openings.

Sources: Coverage arithmetic: covered area = running length × exposed face width (area of a rectangle)

Checked 9 Nov 2026. Coverage arithmetic: covered area = running length × exposed face width (area of a rectangle); 1 m = 100 cm = 1000 mm.

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