Estimate the volume, bags, and cost for your concrete project.
Total Volume Required:0 Cubic Yards
Cubic Feet:0 ft³
60lb Pre-Mix Bags Needed:0 Bags
80lb Pre-Mix Bags Needed:0 Bags
Estimated Bulk Material Cost:$0.00
function calculateConcrete() {
// 1. Get input values
var length = parseFloat(document.getElementById("slabLength").value);
var width = parseFloat(document.getElementById("slabWidth").value);
var thickness = parseFloat(document.getElementById("slabThickness").value);
var waste = parseFloat(document.getElementById("wasteFactor").value);
var price = parseFloat(document.getElementById("pricePerYard").value);
// 2. Validate inputs
if (isNaN(length) || isNaN(width) || isNaN(thickness)) {
alert("Please enter valid numbers for Length, Width, and Thickness.");
return;
}
if (length <= 0 || width <= 0 || thickness 0) {
document.getElementById("resCost").innerHTML = "$" + totalCost.toFixed(2);
} else {
document.getElementById("resCost").innerHTML = "N/A";
}
// Show result box
document.getElementById("resultOutput").style.display = "block";
}
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 wasting money on excess. Concrete is sold by volume, typically in cubic yards for bulk delivery or in pre-mixed bags for smaller DIY projects.
The Concrete Volume Formula
To determine how much concrete you need, you must calculate the volume of the space you are filling. The formula is:
Since concrete thickness is usually measured in inches, you must first convert that dimension to feet by dividing by 12. Once you have the total cubic feet, divide by 27 to convert the result into Cubic Yards, which is the standard unit for ordering from a ready-mix truck.
Standard Thickness Guide
4 Inches: Standard for residential sidewalks, patios, and garage floors. Capable of supporting light vehicle traffic.
5-6 Inches: Recommended for driveways that accommodate heavier vehicles, RVs, or trucks.
For smaller projects where a concrete truck isn't necessary, pre-mixed bags (like Quikrete or Sakrete) are ideal. Here is the yield rule of thumb:
A 60lb bag yields approximately 0.45 cubic feet of concrete.
An 80lb bag yields approximately 0.60 cubic feet of concrete.
Our calculator automatically determines how many bags of each size you would need based on your project's volume.
Why You Need a Waste Factor
A "perfect" pour is rare. Uneven subgrades (the ground beneath the concrete), spillage during the pour, and slight form bowing can all lead to requiring more material than the strict mathematical volume. Professional contractors typically add a 5% to 10% safety margin (waste factor) to ensure they don't run out of concrete in the middle of a job. Running short can result in a "cold joint," which weakens the slab and looks unprofessional.