*Bag counts are rounded up to the nearest whole bag.
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 wastePercent = parseFloat(document.getElementById('wasteFactor').value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thickness) || length <= 0 || width <= 0 || thickness <= 0) {
alert("Please enter valid positive numbers for Length, Width, and Thickness.");
return;
}
// Logic
// 1. Calculate cubic feet (LxWxH). Note: Thickness is in inches, so divide by 12.
var cubicFeetRaw = length * width * (thickness / 12);
// 2. Add Waste Factor
var wasteMultiplier = 1 + (wastePercent / 100);
var totalCubicFeet = cubicFeetRaw * wasteMultiplier;
// 3. Convert to Cubic Yards (1 Yard = 27 Cubic Feet)
var totalCubicYards = totalCubicFeet / 27;
// 4. Calculate Premix Bags
// Standard concrete density is approx 145-150 lbs per cubic foot.
// We will use 133 lbs per cubic foot for dry premix yield calculations (industry standard approximation for yield).
// Actually, a standard rule is:
// One 80lb bag yields approx 0.60 cubic feet.
// One 60lb bag yields approx 0.45 cubic feet.
var bags80 = totalCubicFeet / 0.60;
var bags60 = totalCubicFeet / 0.45;
// Display Results
document.getElementById('resYards').innerText = totalCubicYards.toFixed(2);
document.getElementById('resFeet').innerText = totalCubicFeet.toFixed(2);
document.getElementById('resBags80').innerText = Math.ceil(bags80);
document.getElementById('resBags60').innerText = Math.ceil(bags60);
// Show Result Div
document.getElementById('resultArea').style.display = "block";
}
How to Calculate Concrete for Slabs and Footings
Whether you are pouring a patio, a driveway, or footings for a deck, calculating the correct amount of concrete is the most critical step in planning your project. Ordering too little concrete can result in "cold joints" that weaken the structure, while ordering too much is a waste of money and labor. This guide will help you understand how to use the concrete calculator effectively.
The Concrete Formula
Concrete is measured by volume, specifically in Cubic Yards (often just called "yards"). To find the volume of a rectangular slab, you use the formula:
Volume = Length × Width × Thickness
However, the trick is unit conversion. Length and Width are usually measured in feet, while Thickness is measured in inches. To get an accurate calculation, you must convert the thickness into feet (divide inches by 12) before multiplying.
Understanding Thickness Requirements
Choosing the right thickness depends on what the slab will support:
4 Inches: The standard thickness for residential sidewalks, patios, and garage floors used for passenger cars.
5-6 Inches: Recommended for driveways that hold heavier trucks, RVs, or areas with heavy traffic.
8+ Inches: Heavy-duty industrial floors or commercial foundations.
Why Include a Waste Factor?
Perfect pours are rare. Several factors contribute to needing more concrete than the exact mathematical volume:
Uneven Subgrade: If your dirt base is slightly lower in the middle, you will fill that space with concrete.
Spillage: Some concrete is inevitably lost during the wheelbarrowing and screeding process.
Form Flexing: Wooden forms may bow slightly outward under the weight of wet concrete.
For most DIY projects with a flat grade, a 5% safety margin is sufficient. If you are pouring on uneven ground, increase this to 10%.
Premix Bags vs. Ready-Mix Truck
Once you calculate your total volume, you need to decide how to buy the concrete:
Bagged Concrete: Ideal for projects under 1 cubic yard (approx 45-50 bags of 80lb mix). It requires a mixer and physical labor.
Ready-Mix Truck: Best for any project over 1 cubic yard. It is consistent, easier to pour, and often cheaper for large volumes. Note that many batch plants have a "short load fee" for orders under 4-5 yards.