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How to Calculate Concrete for Your Project
Whether you are pouring a new patio, a driveway, or a foundation for a shed, accurately estimating the amount of concrete required is the most critical step in the planning process. Ordering too little concrete can result in a structural "cold joint" that weakens the slab, while ordering too much wastes money and creates disposal issues.
The Concrete Calculation Formula
To calculate the volume of a concrete slab, you need to determine the area in square feet and multiply it by the depth. However, since dimensions are often measured in feet and inches, the math requires unit conversion. The standard formula used by this calculator is:
Volume (Cu. Ft.) = Length (ft) × Width (ft) × (Depth (in) / 12)
Once you have the cubic footage, you can convert it to Cubic Yards (the standard unit for ordering ready-mix trucks) by dividing by 27.
Understanding Pre-Mix Bags
For smaller DIY projects, you will likely use pre-mixed bags (like Quikrete or Sakrete) rather than ordering a truck. Here is how the calculator estimates the bag count:
- 80lb Bags: The most common size for medium projects. One 80lb bag yields approximately 0.6 cubic feet of cured concrete.
- 60lb Bags: Easier to handle but more expensive per volume. One 60lb bag yields approximately 0.45 cubic feet of cured concrete.
Why Include a Waste Margin?
Professional contractors never order the exact mathematical volume. Several factors contribute to volume loss:
- Uneven Subgrade: If your dirt base is slightly lower in some spots, you will need more concrete to fill the void.
- Spillage: Some material is inevitably lost during mixing and pouring.
- Form Bowing: Wooden forms may bow outward slightly under the weight of wet concrete, increasing the volume.
We recommend a safety margin of 5% to 10% to ensure you can complete the pour in one go.
Common Slab Thicknesses
- 4 Inches: Standard for walkways, patios, and residential garage floors.
- 5-6 Inches: Recommended for driveways that host heavy vehicles or RVs.
- 6+ Inches: Heavy-duty foundations or commercial applications.
function calculateConcrete() {
// Get input values
var len = document.getElementById('slabLength').value;
var wid = document.getElementById('slabWidth').value;
var dep = document.getElementById('slabDepth').value;
var waste = document.getElementById('wastePct').value;
// Validate inputs
if (len === "" || wid === "" || dep === "") {
alert("Please enter values for Length, Width, and Thickness.");
return;
}
// Parse numbers
var lengthFt = parseFloat(len);
var widthFt = parseFloat(wid);
var depthIn = parseFloat(dep);
var wastePct = parseFloat(waste);
if (lengthFt <= 0 || widthFt <= 0 || depthIn <= 0) {
alert("Please enter positive numbers greater than zero.");
return;
}
// 1. Calculate base cubic feet
// Depth needs to be converted from inches to feet (divide by 12)
var depthFt = depthIn / 12;
var cubicFeetBase = lengthFt * widthFt * depthFt;
// 2. Add Waste Margin
var totalCubicFeet = cubicFeetBase * (1 + (wastePct / 100));
// 3. Convert to Cubic Yards (27 cubic feet in 1 cubic yard)
var totalCubicYards = totalCubicFeet / 27;
// 4. Calculate Bags
// Standard yields: 80lb bag = ~0.6 cu ft, 60lb bag = ~0.45 cu ft
var bags80 = Math.ceil(totalCubicFeet / 0.60);
var bags60 = Math.ceil(totalCubicFeet / 0.45);
// Display Results
document.getElementById('resYards').innerText = totalCubicYards.toFixed(2);
document.getElementById('resFeet').innerText = totalCubicFeet.toFixed(2);
document.getElementById('resBags80').innerText = bags80;
document.getElementById('resBags60').innerText = bags60;
// Show result box
document.getElementById('ccResult').style.display = 'block';
}