function calculateConcrete() {
var length = parseFloat(document.getElementById('slabLength').value);
var width = parseFloat(document.getElementById('slabWidth').value);
var thick = parseFloat(document.getElementById('slabThickness').value);
var waste = parseFloat(document.getElementById('wasteFactor').value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thick)) {
alert("Please enter valid numbers for Length, Width, and Thickness.");
return;
}
if (isNaN(waste)) {
waste = 0;
}
// Calculations
// 1. Calculate cubic feet (Length * Width * (Thickness/12))
var thicknessInFeet = thick / 12;
var cubicFeetRaw = length * width * thicknessInFeet;
// 2. Add waste percentage
var totalCubicFeet = cubicFeetRaw * (1 + (waste / 100));
// 3. Convert to Cubic Yards (1 yard = 27 cubic feet)
var totalCubicYards = totalCubicFeet / 27;
// 4. Calculate Bags
// Approx yield: 80lb bag = 0.60 cu ft, 60lb bag = 0.45 cu ft
var bags80 = Math.ceil(totalCubicFeet / 0.60);
var bags60 = Math.ceil(totalCubicFeet / 0.45);
// Output Results
document.getElementById('resCubicYards').innerHTML = totalCubicYards.toFixed(2) + " cu. yd.";
document.getElementById('resCubicFeet').innerHTML = totalCubicFeet.toFixed(2) + " cu. ft.";
document.getElementById('resBags80').innerHTML = bags80 + " bags";
document.getElementById('resBags60').innerHTML = bags60 + " bags";
// Show result div
document.getElementById('csResults').style.display = "block";
}
How to Calculate Concrete for a Slab
Planning a concrete project requires precise measurements to ensure you order the correct amount of material. Whether you are pouring a patio, a driveway, or a shed foundation, underestimating can lead to structural weakness, while overestimating wastes money.
The Concrete Formula
To determine the volume of concrete needed, you must calculate the cubic footage of the area. The formula used is:
Since concrete is typically sold by the Cubic Yard, you must divide the total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).
Standard Slab Thicknesses
4 Inches: The standard for residential sidewalks, patios, and garage floors used for passenger cars.
5-6 Inches: Recommended for driveways that hold heavier vehicles, RVs, or light commercial use.
8+ Inches: Heavy-duty industrial foundations or areas with significant load-bearing requirements.
Why Include a Waste Factor?
Professional contractors always include a "margin of error" or waste factor. This calculator defaults to 5%, which accounts for:
Spillage during the pour.
Uneven subgrade (the ground is rarely perfectly flat).
Settling of the concrete.
For complex shapes or uneven ground, it is safer to increase the waste factor to 10%.
Using Premix Bags vs. Ready-Mix Truck
If your project requires less than 1 cubic yard of concrete (approx. 45-60 bags), it is often more economical to buy premix bags (60lb or 80lb) from a hardware store. For projects requiring more than 2 cubic yards, ordering a ready-mix truck is usually more efficient and cost-effective.