function calculateShiplap() {
// Get Inputs
var hFt = parseFloat(document.getElementById('wallHeightFt').value) || 0;
var hIn = parseFloat(document.getElementById('wallHeightIn').value) || 0;
var wFt = parseFloat(document.getElementById('wallWidthFt').value) || 0;
var wIn = parseFloat(document.getElementById('wallWidthIn').value) || 0;
var exposedWidthIn = parseFloat(document.getElementById('exposedWidth').value);
var boardLengthFt = parseFloat(document.getElementById('boardLength').value);
var wastePercent = parseFloat(document.getElementById('wasteFactor').value);
var price = parseFloat(document.getElementById('pricePerBoard').value) || 0;
// Validation
if (!exposedWidthIn || exposedWidthIn <= 0) {
alert("Please enter a valid exposed board width.");
return;
}
if ((hFt === 0 && hIn === 0) || (wFt === 0 && wIn === 0)) {
alert("Please enter valid wall dimensions.");
return;
}
// 1. Calculate Total Wall Area in Square Feet
var totalHeightInches = (hFt * 12) + hIn;
var totalWidthInches = (wFt * 12) + wIn;
var wallSqFt = (totalHeightInches * totalWidthInches) / 144;
// 2. Calculate Coverage Area per Board (Exposed)
// Convert exposed width to feet
var exposedWidthFt = exposedWidthIn / 12;
var boardCoverageSqFt = exposedWidthFt * boardLengthFt;
// 3. Calculate Raw Boards Needed
var rawBoards = wallSqFt / boardCoverageSqFt;
// 4. Apply Waste Factor
var wasteMultiplier = 1 + (wastePercent / 100);
var boardsWithWaste = rawBoards * wasteMultiplier;
// 5. Round up to nearest whole board
var totalBoards = Math.ceil(boardsWithWaste);
// 6. Calculate Linear Feet
// Linear feet = (Wall Sq Ft / Exposed Width in Ft) * Waste
var totalLinearFeet = Math.ceil((wallSqFt / exposedWidthFt) * wasteMultiplier);
// 7. Calculate Cost
var totalCost = totalBoards * price;
// Display Results
document.getElementById('resArea').innerText = wallSqFt.toFixed(2) + " sq ft";
document.getElementById('resBoards').innerText = totalBoards;
document.getElementById('resLinear').innerText = totalLinearFeet + " ft";
// Format cost currency
var formattedCost = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(totalCost);
document.getElementById('resCost').innerText = formattedCost;
// Show result box
document.getElementById('results').style.display = 'block';
}
How to Calculate Shiplap for Your Walls
Installing shiplap is one of the most effective ways to add texture and farmhouse charm to a room. However, buying the wrong amount of material is a common headache. Under-ordering halts your project, while over-ordering wastes money. This calculator helps you determine exactly how many boards and linear feet you need based on the specific dimensions of your wall and the siding you choose.
1. Measuring Your Wall
Start by measuring the total height and width of the wall in feet and inches. If you have windows or doors, calculate the total area first. For standard rectangular walls, you can simply subtract the square footage of windows and doors from your total result, or simply calculate the full wall and keep the extra material as a safety buffer for cuts and mistakes.
2. The "Exposed Width" Trap
The most critical input in this calculator is the Exposed Board Width. Shiplap boards have a rabbet (a groove) that allows them to overlap.
Total Width: The width of the physical board from edge to edge.
Exposed (Face) Width: The width visible on the wall after installation (Total Width minus the Overlap).
For example, a nominal 6-inch shiplap board might have a total width of 5.5 inches, but an exposed face of only 5.0 or 5.25 inches. Always calculate using the exposed width, or you will run short on materials.
3. Determining Waste Factor
You cannot simply buy the exact square footage of the wall. You must account for "waste"—material lost when cutting boards to fit the wall length, trimming ends, or discarding defective pieces.
10% Waste: Standard for simple rectangular walls with minimal outlets or obstacles.
15-20% Waste: Recommended if you are installing the shiplap diagonally, in a herringbone pattern, or in a room with many windows, doors, and angles where intricate cuts are required.
4. Cost Estimation
Once you know the total number of boards required (rounded up to the nearest whole board), multiply this by the price per board to get your material estimate. Remember to factor in the cost of nails, adhesive, and trim pieces, which are not included in the raw board cost.
Installation Tip: The Nickel Gap
Many modern shiplap installations feature a "nickel gap"—a small space between boards. If you are spacing your boards manually (using a spacer) rather than relying on a self-spacing milled profile, add that gap size to your "Exposed Width" input. For example, if your board is 5″ wide and you leave a 1/8″ gap, your coverage width is 5.125″.