Sandstone/Limestone (150 lbs/ft³)
Granite (165 lbs/ft³)
Basalt/Trap Rock (175 lbs/ft³)
Standard boulder walls have 10-20% air gaps/overlapping.
Estimated Materials Needed:
0 Tons
Total Weight: 0 lbs
Wall Volume: 0 Cubic Feet
How to Estimate Boulders for Your Retaining Wall
Planning a boulder wall requires more than just picking pretty rocks; it requires an understanding of volume and mass. Because boulders are irregular, calculating the exact quantity involves estimating the cubic volume of the wall and applying a density factor based on the type of stone you choose.
The Calculation Formula
To find the tonnage required for a boulder wall, we use the following engineering formula:
1. Depth and Stability: For structural retaining walls, the depth (thickness) of the wall should generally be 50% to 70% of the wall's height. If your wall is 4 feet tall, your boulders should be at least 2 to 3 feet deep to provide gravity-based stability.
2. Rock Density: Not all stone weighs the same. Granite is significantly denser and heavier than sandstone. Choosing a heavier stone means you will need more tons for the same physical volume.
3. The "Air Gap" Factor: Unlike a poured concrete wall, a boulder wall has gaps between the rocks. While you pay for the weight of the stone, the volume it fills includes these gaps. Our calculator includes a "Buffer" setting to account for this stacking efficiency.
Example Calculation
If you are building a granite wall that is 30 feet long, 4 feet high, and 2 feet deep:
• Step 4: Adding a 15% buffer for waste/gaps = ~22.77 Tons.
function calculateBoulders() {
var length = parseFloat(document.getElementById('wallLength').value);
var height = parseFloat(document.getElementById('wallHeight').value);
var depth = parseFloat(document.getElementById('wallDepth').value);
var density = parseFloat(document.getElementById('rockType').value);
var buffer = parseFloat(document.getElementById('buffer').value);
if (isNaN(length) || isNaN(height) || isNaN(depth) || length <= 0 || height <= 0 || depth <= 0) {
alert("Please enter valid positive dimensions for the wall.");
return;
}
if (isNaN(buffer)) { buffer = 0; }
// Volume in cubic feet
var volume = length * height * depth;
// Weight in pounds
var baseWeight = volume * density;
// Apply buffer (wastage/air gaps)
// Note: Usually buffer is added to account for the fact that you buy by weight
// but the volume includes gaps. However, for stone, you usually order 5-10% extra
// to ensure you have enough 'fitting' pieces.
var totalWeightLbs = baseWeight * (1 + (buffer / 100));
var totalTons = totalWeightLbs / 2000;
// Display Results
document.getElementById('totalVolume').innerText = volume.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('totalLbs').innerText = Math.round(totalWeightLbs).toLocaleString();
document.getElementById('totalTons').innerText = totalTons.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resultArea').style.display = 'block';
// Smooth scroll to result
document.getElementById('resultArea').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}