Calculate cubic yards, premix bags, and costs for your project.
ft
ft
in
%
$
Project Calculation Results
Total Volume (Cubic Feet):–
Total Volume (Cubic Yards):–
Estimated Ready Mix Cost:–
Premix Bag Estimator (DIY)
80lb Bags Required:–
60lb Bags Required:–
*Calculations include your selected waste margin.
function calculateConcrete() {
// Get input values
var length = parseFloat(document.getElementById('slabLength').value);
var width = parseFloat(document.getElementById('slabWidth').value);
var thickness = parseFloat(document.getElementById('slabThickness').value);
var quantity = parseFloat(document.getElementById('slabQuantity').value);
var waste = parseFloat(document.getElementById('wasteFactor').value);
var price = parseFloat(document.getElementById('pricePerYard').value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thickness) || length <= 0 || width <= 0 || thickness <= 0) {
alert("Please enter valid positive numbers for dimensions.");
return;
}
if (isNaN(quantity) || quantity < 1) quantity = 1;
if (isNaN(waste) || waste 0) {
var totalCost = totalCuYd * price;
document.getElementById('resCost').innerHTML = "$" + totalCost.toFixed(2);
document.getElementById('costRow').style.display = 'flex';
} else {
document.getElementById('costRow').style.display = 'none';
}
// Show result div
document.getElementById('result').style.display = 'block';
}
How to Calculate Concrete for Slabs and Patios
Planning a new patio, driveway, or shed foundation requires precise measurements to ensure you order enough material without overspending. This Concrete Slab Calculator helps you determine exactly how much concrete you need in both cubic yards (for ready-mix trucks) and bags (for DIY projects).
Pro Tip: Always include a "Waste Margin" of 5-10% in your calculations. Spillage, uneven subgrades, and form bending often require more concrete than a perfect mathematical volume calculation suggests.
The Formula for Concrete Volume
To calculate the volume of a concrete slab, you use the standard volume formula: Length × Width × Height. However, because concrete is sold by the Cubic Yard but measured in feet and inches, conversion is necessary.
Step 1: Convert thickness from inches to feet (divide by 12).
Step 2: Multiply Length (ft) × Width (ft) × Thickness (ft) to get Cubic Feet.
Step 3: Divide Cubic Feet by 27 to get Cubic Yards.
How Many Bags of Concrete Do I Need?
If you are mixing the concrete yourself using pre-mixed bags (like Quikrete or Sakrete), you need to know the yield of the bags rather than just 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 automatically divides your required total cubic footage by these yields to tell you exactly how many bags to buy.
Standard Slab Thickness Guidelines
The depth of your concrete depends heavily on its intended use:
4 Inches: Standard for walkways, patios, and residential shed floors.
5-6 Inches: Recommended for driveways carrying standard passenger vehicles.
6+ Inches: Required for heavy equipment or RV parking pads.
Ready Mix vs. Bagged Concrete
When should you order a truck versus buying bags? A general rule of thumb is the 1-yard rule. If your project requires more than 1 cubic yard of concrete (about 45 bags of 80lb mix), it is often more economical and physically easier to order a ready-mix truck delivery. For projects under 1 yard, mixing bags yourself is usually cheaper.