.calculator-container {
max-width: 800px;
margin: 0 auto;
font-family: 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
border: 1px solid #e0e0e0;
border-radius: 8px;
padding: 20px;
background-color: #ffffff;
box-shadow: 0 2px 10px rgba(0,0,0,0.05);
}
.calc-header {
text-align: center;
margin-bottom: 25px;
background-color: #2c3e50;
color: white;
padding: 15px;
border-radius: 6px;
}
.calc-row {
display: flex;
flex-wrap: wrap;
gap: 20px;
margin-bottom: 20px;
}
.calc-col {
flex: 1;
min-width: 200px;
}
.calc-label {
display: block;
margin-bottom: 8px;
font-weight: 600;
color: #333;
}
.calc-input {
width: 100%;
padding: 12px;
border: 1px solid #ccc;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
}
.calc-btn {
display: block;
width: 100%;
padding: 15px;
background-color: #e67e22;
color: white;
border: none;
border-radius: 4px;
font-size: 18px;
font-weight: bold;
cursor: pointer;
transition: background-color 0.3s;
margin-top: 10px;
}
.calc-btn:hover {
background-color: #d35400;
}
.calc-results {
margin-top: 30px;
padding: 20px;
background-color: #f8f9fa;
border-left: 5px solid #27ae60;
display: none;
}
.result-item {
display: flex;
justify-content: space-between;
margin-bottom: 12px;
padding-bottom: 12px;
border-bottom: 1px solid #eee;
}
.result-item:last-child {
border-bottom: none;
}
.result-label {
color: #555;
font-weight: 500;
}
.result-value {
font-weight: 700;
color: #2c3e50;
}
.article-section {
margin-top: 40px;
line-height: 1.6;
color: #444;
}
.article-section h2 {
color: #2c3e50;
border-bottom: 2px solid #eee;
padding-bottom: 10px;
margin-top: 30px;
}
.article-section h3 {
color: #e67e22;
margin-top: 25px;
}
.article-section ul {
padding-left: 20px;
}
.highlight-box {
background-color: #e8f4f8;
padding: 15px;
border-radius: 5px;
margin: 20px 0;
}
How to Calculate Concrete for Slabs and Patios
Planning a new patio, driveway, or shed foundation requires precise calculations to ensure you order enough material without overspending. This Concrete Slab Calculator helps you determine exactly how much premixed concrete or ready-mix you need based on the dimensions of your project.
Pro Tip: Concrete volume is measured in Cubic Yards. One cubic yard of concrete weighs approximately 3,600 – 4,000 lbs, depending on the aggregate mix.
The Concrete Calculation Formula
To calculate the concrete volume manually, you must convert all dimensions to the same unit (usually feet) before determining the volume.
Step 1: Convert Thickness to Feet
Since slabs are measured in inches of thickness but feet of length/width, divide the inches by 12.
Example: 4 inches / 12 = 0.33 feet.
Step 2: Calculate Cubic Feet
Multiply Length × Width × Thickness (in feet).
Formula: Volume (ft³) = L × W × T
Step 3: Convert to Cubic Yards
There are 27 cubic feet in 1 cubic yard. Divide your total cubic feet by 27.
Formula: Cubic Yards = Volume (ft³) / 27
How Many Bags of Concrete Do I Need?
If you are using pre-mixed bags (like Quikrete or Sakrete) from a hardware store, the calculation depends on the bag size:
- 80lb Bags: Yield approximately 0.6 cubic feet.
- 60lb Bags: Yield approximately 0.45 cubic feet.
Our calculator uses a standard density of roughly 133 lbs per cubic foot to estimate the exact bag count required for your project dimensions.
Why Include a Waste Margin?
It is critical to include a "waste margin" or safety factor—typically 5% to 10%—when ordering concrete. This accounts for:
- Spillage during the pour.
- Uneven subgrade (ground) depth.
- Settling of forms.
- Some concrete remaining in the mixer.
Running out of concrete mid-pour creates a "cold joint," which weakens the slab and looks unsightly. It is always cheaper to buy an extra bag than to halt a project.
function calculateConcrete() {
// Get input values
var length = parseFloat(document.getElementById('slabLength').value);
var width = parseFloat(document.getElementById('slabWidth').value);
var thicknessInches = parseFloat(document.getElementById('slabThickness').value);
var wastePercent = parseFloat(document.getElementById('wasteMargin').value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thicknessInches)) {
alert("Please enter valid numbers for Length, Width, and Thickness.");
return;
}
if (length <= 0 || width <= 0 || thicknessInches <= 0) {
alert("Dimensions must be greater than zero.");
return;
}
// Default waste to 0 if empty
if (isNaN(wastePercent)) {
wastePercent = 0;
}
// 1. Convert thickness to feet
var thicknessFeet = thicknessInches / 12;
// 2. Calculate Cubic Feet
var cubicFeet = length * width * thicknessFeet;
// 3. Add Waste Margin
var totalCubicFeet = cubicFeet * (1 + (wastePercent / 100));
// 4. Convert to Cubic Yards (27 cubic feet = 1 cubic yard)
var cubicYards = totalCubicFeet / 27;
// 5. Calculate Bags
// Standard concrete density is approx 133 lbs per cubic foot
// 80lb bag yields approx 0.6 cu ft (80/133 = 0.601)
// 60lb bag yields approx 0.45 cu ft (60/133 = 0.451)
// Using weight method is more accurate:
var totalWeightLbs = totalCubicFeet * 133;
var bags80 = Math.ceil(totalWeightLbs / 80);
var bags60 = Math.ceil(totalWeightLbs / 60);
// Formatting outputs
document.getElementById('resCubicYards').innerText = cubicYards.toFixed(2) + " yd³";
document.getElementById('resCubicFeet').innerText = totalCubicFeet.toFixed(2) + " ft³";
document.getElementById('resBags80').innerText = bags80 + " Bags";
document.getElementById('resBags60').innerText = bags60 + " Bags";
// Show results
document.getElementById('resultsArea').style.display = "block";
}