Shiplap Calculator - Wall & Ceiling Material Estimator

What Is a Shiplap Calculator?

A shiplap calculator is a tool that works out how much shiplap board is needed to cover a wall or ceiling. It multiplies the length and width of the surface to find the area, then adds extra material to cover cuts and mistakes made during installation. Shiplap itself is a type of wooden board with a rabbeted (notched) edge. Each board overlaps the one below it, leaving a small horizontal groove called a reveal. Builders once used shiplap boards as exterior siding because the overlap sheds rainwater. Today the same board style is common on interior accent walls and ceilings.

Shiplap Calculator Formula

The calculator uses two steps.

Step 1: Find the area.

Area = Length × Width

Step 2: Add a waste allowance.

Shiplap needed = Area × 1.10

The waste allowance is fixed at 10%. It covers boards lost to cutting, damaged pieces, and small measuring errors. The calculator does not let a user change this percentage, and it does not subtract the area of windows or doors — the result is the total shiplap for the full length and width entered.

How to Use the Calculator

1. Choose a unit of measurement, feet or meters. This only changes the label on the results; it does not convert numbers already typed into the length or width fields. Enter dimensions in the unit already selected before switching.
2. Enter the length of the surface.
3. Enter the width. For a wall, use its height as the width value.
4. Read the results: total area and the amount of shiplap needed, both rounded to two decimal places.
5. Copy the results with the copy button if needed.

Worked Example

A wall measures 12 feet long and 8 feet high.

• Area = 12 × 8 = 96 square feet
• Shiplap needed = 96 × 1.10 = 105.6 square feet

A second example: a bedroom ceiling measures 12 by 14 feet.

• Area = 12 × 14 = 168 square feet
• Shiplap needed = 168 × 1.10 = 184.8 square feet

The calculator always applies the same 10% allowance shown above, whether the surface is a wall or a ceiling. Some installers buy extra boards beyond that figure for surfaces with many corner or angle cuts, such as ceilings, since those cuts waste more material than a plain wall.

To estimate every wall in a room at once, add the lengths of all four walls together and enter that total as the length, with the ceiling height as the width. For a 12-by-12-foot room with 8-foot ceilings, the four walls total 48 feet of length: 48 × 8 = 384 square feet of area, and 384 × 1.10 = 422.4 square feet of shiplap. This total does not subtract windows or doors.

Why Shiplap Needs a Waste Allowance

Every board installation loses some material. Boards get cut short to fit around corners, some pieces arrive with defects, and installers make occasional measuring mistakes. Construction estimators commonly add 10% to 20% extra material for wall coverings, with more added for jobs that have many cuts, such as rooms with several windows, angled ceilings, or exterior siding. This calculator uses a flat 10% for every calculation. A project with unusually complex cuts may need more material than the calculator shows.

Shiplap Compared With Other Wall Coverings

• Tongue and groove: boards interlock with a protruding edge that fits into a groove on the next board, giving a tighter seal than shiplap's overlapping edges. It is common in bathrooms and other damp rooms.
• Board and batten: narrow strips (battens) cover the seams between wider boards, creating raised vertical lines instead of shiplap's horizontal reveal.
• Beadboard: narrow, closely spaced boards with a rounded groove between them, traditionally used for wainscoting.

The area formula above works for all of these, since each covers a surface with material bought by the square foot or square meter. Only the waste allowance a person chooses to add should change between them.

Frequently Asked Questions

What is the formula for shiplap needed? Multiply length by width to get the area, then multiply that area by 1.10 to add a 10% waste allowance.

Does the calculator subtract windows and doors? No. It returns the shiplap needed for the full length and width entered. A person estimating a wall with a large window should reduce the length or width manually, or accept that the result includes some extra material for the opening.

Why does switching between feet and meters not change my numbers? The unit selector only changes the label shown next to the result. It does not convert the length and width values already entered. Enter figures in the unit currently selected.

Is 10% always enough waste allowance? It is enough for most flat walls with few cuts. Ceilings, rooms with several windows or doors, and exterior siding usually need more material than the 10% figure covers, because more cuts mean more waste.

What is the difference between shiplap and tongue and groove? Shiplap boards have a rabbeted edge that overlaps the next board, leaving a visible gap called a reveal. Tongue and groove boards interlock with a protruding tongue that fits into a groove, closing the seam almost completely.

Can this calculator be used for a full room instead of one wall? Yes. Add the lengths of all the walls together and enter that sum as the length, with the ceiling height as the width, to get the total area and shiplap needed for the whole room.