What this shiplap calculator actually estimates
This calculator is built for one rectangular wall or another uninterrupted cladding field. Instead of treating shiplap as generic square footage, it first works out how many horizontal courses or vertical runs the wall needs from the board's real exposed face. It then converts that layout into linear footage, waste and an estimated stock-board quantity.
- wall area
- horizontal course count or vertical run count
- visible coverage required in the final course
- installed linear feet or metres before waste
- linear material including the selected allowance
- minimum pieces per course from the selected stock length
- minimum required butt joints from stock length and estimated complete stock boards
The key input is exposed face or coverage width. Do not enter the nominal board size unless the manufacturer specifically says that value is the installed coverage. The rabbet, tongue or reveal can make the usable width noticeably smaller.
Why exposed face width matters more than nominal board size
Shiplap boards overlap or interlock along their edges. Part of the milled board therefore disappears behind the next piece, so the width that adds new wall coverage is smaller than the number printed in a nominal 1×6 or 1×8 description. Using nominal width is one of the easiest ways to under-order a project.
Which board width belongs in the calculator?
| Term | What it means | Use it here? |
|---|---|---|
| Nominal width | The trade size used in the product name, such as 1×6 | No |
| Actual width | The full milled width measured edge to edge | Not necessarily |
| Exposed face / coverage width | The width that remains as new visible coverage after installation | Yes — this is the main value |
| Reveal / gap | The visible joint line between boards | Only when the product documentation uses it to define net coverage |
Manufacturers often publish exposed face separately from actual width for exactly this reason. If the product page gives a coverage area per piece instead, divide that coverage area by the board length to recover the effective coverage width before calculating the wall.
Only four main inputs
The main form is intentionally short. For a useful first estimate you only need board orientation, wall width, wall height and exposed coverage width. Stock length and waste are available under Advanced settings with practical defaults, so a homeowner can get a result without filling in a contractor-style material schedule.
Shiplap calculator inputs
| Input | What it controls | Best source |
|---|---|---|
| Board orientation | Whether courses are counted up the wall or runs are counted across it | Your planned design |
| Wall width | Horizontal run length or width available for vertical runs | Measure the finished cladding field |
| Wall height | Number of horizontal courses or vertical board length | Measure between the real top and bottom boundaries |
| Exposed face / coverage width | How much new wall width each installed board adds | Manufacturer specification or measured assembled sample |
| Stock board length | How many pieces may be needed in each run | Supplier stock length |
| Waste allowance | Extra material for cuts, culls and mistakes | Project complexity and your cutting plan |
Horizontal shiplap calculation
For horizontal shiplap, wall height determines the number of courses. The calculator divides the height by exposed coverage width and rounds up because a partial final course still requires another board run. Every course then spans the wall width.
Courses = ceil(wall height ÷ exposed coverage width)
Installed linear length = courses × wall width
Linear length with waste = installed linear length × (1 + waste ÷ 100)Vertical shiplap calculation
Vertical shiplap uses the same geometry turned through 90 degrees. Wall width determines the number of vertical runs, and wall height becomes the length each run has to cover. This can materially change the stock length, seam count and amount of cutting even when the wall area stays exactly the same.
Vertical runs = ceil(wall width ÷ exposed coverage width)
Installed linear length = runs × wall heightThe final course is a layout check, not just another number
Most walls are not an exact multiple of one board's coverage. The final course may therefore be almost full width or become a narrow strip. The calculator shows the visible coverage still required at that edge. It does not claim that this is the physical rip width of the board, because rabbets and tongue-and-groove profiles have different edge geometry.
If the final visible strip is very narrow, many installers prefer to share the rip between the first and last courses for a more balanced appearance. Only do that when the chosen profile and fixing method allow the edge detail to be modified safely.
Worked example: 12 ft × 8 ft wall
How much horizontal shiplap is needed for a 12 ft wide × 8 ft high wall using a 4.625 in exposed face, 8 ft stock boards and 10% waste?
Answer: Eight feet is 96 in. Dividing 96 by 4.625 gives 20.76, so the wall needs 21 courses. Each course is 12 ft long, making the installed linear requirement 21 × 12 = 252 ft. Adding 10% produces 277.2 linear ft. With 8 ft stock, that is an estimated 35 complete boards by total length. The last course has about 3.5 in of visible wall left to cover.
Explanation: The calculator rounds the course count before calculating linear material. That reflects the real layout better than assuming every square foot of a partly used final board can be transferred somewhere else.
Stock board length, seams and board count
When the wall run is longer than one stock board, each course needs more than one piece. The calculator reports the minimum number of pieces per course and the minimum butt joints forced by a stock length shorter than the run. Stock boards to buy are estimated from total linear material including waste and assume that usable offcuts can be reused efficiently where the installation rules allow.
A quantity estimate does not decide where a butt joint is permitted. Many shiplap systems require joints to land on framing, furring or another approved backing. Follow the selected product instructions for joint support, fasteners and seam staggering rather than allowing a joint to float simply because the arithmetic fits.
Why there is no simple “subtract doors and windows” field
A rectangular opening does not reduce a board layout in the same way that it reduces paint area. Courses still continue above, below and beside a window or door, and the offcuts may or may not be reusable. Subtracting the opening's square footage can therefore make a board order look more precise while actually making it less reliable.
- For a small opening, calculate the full rectangular wall and let the extra material contribute to the cutting allowance.
- For a large door or window, divide the wall into simple rectangular zones and calculate the zones separately.
- For a project where seam positions matter, make a wall elevation showing openings and framing before the final order.
How much shiplap waste should you add?
Waste depends more on the cutting plan than on one universal percentage. Straight, uninterrupted walls use material efficiently. Short returns, several openings, rejected boards, damaged ends and repeated butt joints can make offcuts harder to reuse.
Practical starting allowances
| Project | Starting allowance | What can increase it |
|---|---|---|
| Simple straight wall | 5–10% | End cuts, damaged boards, small measurement differences |
| Several openings or repeated seams | 10–15% | More short pieces and fewer reusable offcuts |
| Complex corners or irregular cladding field | 15%+ | Layout-specific cuts and rejected pieces |
Shiplap is not the same calculation as slat walls or wainscoting
Shiplap creates a continuous field of overlapping or interlocking boards. A wood slat wall is intentionally open between individual slats, while board and batten, wainscoting and picture-frame molding are spacing-and-trim layouts. HomDera keeps those intents separate so this calculator can stay focused on board coverage instead of becoming a complicated generic wall-panel form.
Planning separate decorative slats? Use the Wood Slat Wall CalculatorFor framed lower-wall panels, use the Wainscoting CalculatorCommon shiplap estimating mistakes
Mistake and better check
| Mistake | What goes wrong | Better approach |
|---|---|---|
| Using nominal board width | Course count is too low | Use the published exposed face or net coverage width |
| Using area only | The final partial course disappears inside the square-foot math | Count courses first, then linear material |
| Ignoring orientation | A vertical layout is treated like a horizontal one | Choose the real board direction before calculating |
| Subtracting every opening by area | The order may be reduced more than the cutting plan allows | Calculate large wall zones separately |
| Ignoring stock length | The material total hides repeated butt joints | Compare the wall run with available board lengths |
| Assuming every offcut is reusable | Board count becomes too optimistic | Keep a sensible waste allowance and respect seam support rules |
Shiplap calculator FAQ
What if the product only lists actual width?
Do not automatically use actual width. Look for exposed face, coverage width, reveal coverage or coverage area per piece. If coverage area and board length are published, divide the area by length to derive the effective coverage width.
Can I use this calculator for tongue-and-groove boards?
The quantity method also works for many tongue-and-groove wall or ceiling products when you know the true installed coverage width. Product suitability, expansion gaps, fixing method and joint details still come from the manufacturer rather than this calculator.
Can I use it for a shiplap ceiling?
You can use the rectangular geometry for a material estimate by entering the ceiling field dimensions. The calculator does not verify joist direction, backing, fastening, board weight or overhead-installation requirements, so those checks remain separate.
Is this an exterior siding calculator?
No. The quantity geometry may look similar, but this tool is aimed at interior shiplap and wall cladding. Exterior siding needs project-specific exposure, water-resistive layers, flashing, ventilation, corner details and fastening requirements that are outside this estimate.
Why can the estimated stock-board count differ from the number a carpenter orders?
The stock-board result converts the calculated linear material into complete lengths and assumes reasonable reuse of offcuts. A real cut plan may require more boards because seams must land on support, boards have defects, colour or grain is selected, or short offcuts cannot be used in the next course.
Use the result as a purchase check
Start with the course count and final-course width to confirm the layout. Then use linear footage including waste to compare different stock lengths. Treat the stock-board quantity as a purchasing estimate rather than a cut map. Before ordering, verify the exact exposed face, available lengths, pack quantities, seam-support requirements and installation instructions for the product you will actually install.