Note: Bag counts are rounded up to the nearest whole bag. Volumes include your selected waste margin.
function calculateConcrete() {
var lengthInput = document.getElementById('slabLength');
var widthInput = document.getElementById('slabWidth');
var thickInput = document.getElementById('slabThickness');
var priceInput = document.getElementById('pricePerYard');
var wasteInput = document.getElementById('wasteFactor');
var errorDiv = document.getElementById('errorDisplay');
var resultDiv = document.getElementById('calcResults');
// Reset display
errorDiv.style.display = "none";
resultDiv.style.display = "none";
// Parse Values
var length = parseFloat(lengthInput.value);
var width = parseFloat(widthInput.value);
var thickness = parseFloat(thickInput.value);
var price = parseFloat(priceInput.value);
var waste = parseFloat(wasteInput.value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thickness) || length <= 0 || width <= 0 || thickness 0) {
totalCost = cubicYards * price;
document.getElementById('costRow').style.display = "flex";
document.getElementById('resCost').innerHTML = "$" + totalCost.toFixed(2);
} else {
document.getElementById('costRow').style.display = "none";
}
// Output Results
document.getElementById('resYards').innerHTML = cubicYards.toFixed(2) + " Cubic Yards";
document.getElementById('resFeet').innerHTML = cubicFeetTotal.toFixed(2) + " ft³";
document.getElementById('res80bags').innerHTML = bags80 + " Bags";
document.getElementById('res60bags').innerHTML = bags60 + " Bags";
resultDiv.style.display = "block";
}
How to Calculate Concrete for a Slab
Whether you are pouring a patio, a driveway, or a foundation slab, accurate calculation of concrete volume is critical to the success of your project. Ordering too little concrete creates stressful "cold joints" and weakens the structure, while ordering too much is a waste of money.
The Concrete Formula
To determine how much concrete you need, you must calculate the volume of the space in cubic yards. The standard formula is:
Step 1: Measure the Length and Width in feet.
Step 2: Measure the Thickness in inches and convert to feet (divide by 12).
Step 3: Multiply Length × Width × Height to get Cubic Feet.
Step 4: Divide Cubic Feet by 27 to get Cubic Yards.
Should You Mix Bags or Order a Truck?
Our concrete calculator provides results for both ready-mix trucks (Cubic Yards) and pre-mix bags (60lb or 80lb). As a general rule of thumb:
Small Projects (under 1 yard): Use bags. This is manageable for a wheelbarrow and hoe or a small electric mixer. Examples include post holes, small landings, or sidewalks.
Large Projects (over 1 yard): Order a ready-mix truck. One cubic yard of concrete weighs approximately 4,000 lbs. Trying to mix 45+ bags of concrete by hand is labor-intensive and makes it difficult to achieve a consistent finish before the concrete sets.
Standard Slab Thicknesses
Choosing the right thickness depends on the load the slab will bear:
4 Inches: Standard for residential sidewalks, patios, and garage floors for passenger cars.
5-6 Inches: Recommended for driveways holding heavier trucks, RVs, or heavy machinery.
6+ Inches: Heavy-duty industrial foundations.
Why Include a Waste Margin?
Professional contractors always order slightly more concrete than the exact mathematical volume. This accounts for spillage, uneven subgrade (dips in the ground), and form spreading. A standard safety margin is 5% to 10%. This calculator allows you to select your preferred waste factor to ensure you don't run short during the pour.