Recommended: 5-10% for spills and uneven subgrade.
Please enter valid positive numbers for dimensions.
Total Volume (Cubic Yards):0
Total Volume (Cubic Feet):0
80lb Bags (Pre-mix):0
60lb Bags (Pre-mix):0
*Calculations include the specified waste margin.
function calculateConcrete() {
// 1. Get input values using specific IDs
var len = document.getElementById('slabLength').value;
var wid = document.getElementById('slabWidth').value;
var thick = document.getElementById('slabThickness').value;
var waste = document.getElementById('wasteMargin').value;
var resultDiv = document.getElementById('csResult');
var errorDiv = document.getElementById('csErrorMessage');
// 2. Validate inputs
if (len === "" || wid === "" || thick === "" || isNaN(len) || isNaN(wid) || isNaN(thick)) {
errorDiv.style.display = 'block';
resultDiv.style.display = 'none';
return;
}
// Parse floats to ensure math works correctly
var l = parseFloat(len);
var w = parseFloat(wid);
var t = parseFloat(thick);
var wm = parseFloat(waste);
if (l <= 0 || w <= 0 || t <= 0) {
errorDiv.style.display = 'block';
resultDiv.style.display = 'none';
return;
}
// Hide error if validation passes
errorDiv.style.display = 'none';
// 3. Perform Calculations
// Formula: Length (ft) * Width (ft) * (Thickness (in) / 12) = Cubic Feet
var thicknessInFeet = t / 12;
var cubicFeet = l * w * thicknessInFeet;
// Add Waste Margin
var wasteMultiplier = 1 + (wm / 100);
var totalCubicFeet = cubicFeet * wasteMultiplier;
// Convert to Cubic Yards (27 cubic feet in 1 cubic yard)
var totalCubicYards = totalCubicFeet / 27;
// Calculate Bags
// Approx yield: 80lb bag = 0.60 cubic feet
// Approx yield: 60lb bag = 0.45 cubic feet
var bags80 = Math.ceil(totalCubicFeet / 0.60);
var bags60 = Math.ceil(totalCubicFeet / 0.45);
// 4. Update the DOM
document.getElementById('resFeet').innerHTML = totalCubicFeet.toFixed(2) + " ft³";
document.getElementById('resYards').innerHTML = totalCubicYards.toFixed(2) + " yd³";
document.getElementById('resBags80').innerHTML = bags80 + " bags";
document.getElementById('resBags60').innerHTML = bags60 + " bags";
// Show result container
resultDiv.style.display = 'block';
}
How to Calculate Concrete for Your Slab
Planning a patio, driveway, or shed foundation requires accurate material estimation. Ordering too little concrete leads to expensive delays and "cold joints," while ordering too much is a waste of money. This Concrete Slab Calculator helps you determine exactly how many cubic yards or pre-mix bags you need for your project.
The Concrete Volume Formula
To calculate the concrete volume, you treat the slab as a rectangular prism. The math requires converting all dimensions to the same unit (usually feet) before multiplying.
Once you have the cubic footage, it is standard construction practice to convert this to Cubic Yards, as this is how ready-mix trucks measure delivery.
1 Cubic Yard = 27 Cubic Feet.
Estimating Bags of Concrete (60lb vs 80lb)
For smaller DIY projects where a truck delivery isn't feasible, you will likely purchase pre-mixed bags (like Quikrete or Sakrete) from a hardware store. The yield of these bags depends on the weight:
80lb Bag: Yields approximately 0.60 cubic feet of cured concrete.
60lb Bag: Yields approximately 0.45 cubic feet of cured concrete.
Our calculator rounds up to the nearest whole bag to ensure you don't run out mid-pour.
Why Include a Waste Margin?
Perfect pours are rare. We recommend adding a 5% to 10% safety margin (or "waste factor") to your calculation. This accounts for:
Spillage during the pour.
Uneven subgrade (dips in the ground make the slab thicker in spots).
Concrete remaining inside the mixer or wheelbarrow.
Standard Slab Thicknesses
Not sure how thick your slab should be? Here are common standards:
4 Inches: Standard for walkways, patios, and residential garage floors.
5-6 Inches: Required for driveways holding heavy vehicles or RVs.