Estimate Volume, Bags, and Material Costs Instantly
Planning a new driveway, patio, or sidewalk? Accurately estimating the amount of concrete you need is the first step to a successful project. Ordering too little can result in catastrophic "cold joints" in your slab, while ordering too much wastes budget on disposal fees.
This Concrete Slab Calculator helps you determine exactly how many cubic yards of concrete or premix bags (80lb/60lb) you need based on your project's dimensions. It includes a standard waste factor to ensure you have enough material to account for uneven subgrades and spillage.
Enter Project Dimensions
Calculation Results (with 5% waste included)
Total Volume Required:0 Cubic Yards
Volume in Cubic Feet:0 ft³
Est. Total Weight:0 lbs
Estimated Cost:$0.00
For DIY (Pre-Mix Bags)
80lb Bags Needed:0 bags
60lb Bags Needed:0 bags
How to Calculate Concrete Volume
The basic formula for calculating concrete volume is Length × Width × Thickness. However, since concrete is sold by the Cubic Yard (yd³), you must convert your measurements to ensure they are consistent.
The Formula:
1. Convert thickness from inches to feet (divide by 12).
2. Multiply Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet.
3. Divide Cubic Feet by 27 = Cubic Yards.
Standard Thickness Guide
Choosing the right thickness is vital for the longevity of your slab:
4 Inches: Standard for residential sidewalks, patios, and garage floors for passenger vehicles.
5-6 Inches: Recommended for driveways that hold heavier trucks, RVs, or endure freeze-thaw cycles.
6+ Inches: Heavy-duty commercial aprons or areas with heavy machinery.
Understanding the Waste Factor
It is standard industry practice to order 5% to 10% more concrete than your exact mathematical calculation. This accounts for:
Spillage during the pour.
Uneven subgrade (the ground is rarely perfectly flat).
Concrete sticking to the inside of the mixer truck or wheelbarrows.
Our calculator defaults to a 5% safety margin, but you can adjust this based on the complexity of your site.
Frequently Asked Questions
How many bags of concrete make a yard?
One cubic yard of concrete weighs approximately 3,600 to 4,000 lbs depending on the aggregate. As a general rule of thumb, you will need approximately:
45 bags of 80lb pre-mix concrete for 1 cubic yard.
60 bags of 60lb pre-mix concrete for 1 cubic yard.
Should I mix by hand or order a truck?
If your project requires more than 1 cubic yard (roughly 45 bags of 80lb mix), it is usually more cost-effective and physically manageable to order a ready-mix truck. Hand-mixing this volume often leads to inconsistent curing times across the slab.
function calculateConcrete() {
// 1. Get Input Values
var len = parseFloat(document.getElementById('slabLength').value);
var wid = parseFloat(document.getElementById('slabWidth').value);
var thick = parseFloat(document.getElementById('slabThickness').value);
var qty = parseFloat(document.getElementById('slabQuantity').value);
var cost = parseFloat(document.getElementById('costPerYard').value);
var waste = parseFloat(document.getElementById('wasteFactor').value);
// 2. Validate Inputs
if (isNaN(len) || isNaN(wid) || isNaN(thick) || isNaN(qty)) {
alert("Please enter valid numbers for Length, Width, Thickness, and Quantity.");
return;
}
// Handle optional inputs (defaults)
if (isNaN(cost)) cost = 0;
if (isNaN(waste)) waste = 0;
// 3. Perform Calculations
// Convert thickness to feet
var thickFeet = thick / 12;
// Calculate base Cubic Feet
var cubicFeet = len * wid * thickFeet * qty;
// Apply Waste Factor
var wasteMultiplier = 1 + (waste / 100);
var totalCubicFeet = cubicFeet * wasteMultiplier;
// Convert to Cubic Yards
var totalCubicYards = totalCubicFeet / 27;
// Calculate Weight (Approx 145 lbs per cubic foot for cured concrete, approx 3915 lbs/yd)
// Using standard 150lb/ft3 for wet concrete estimation
var totalWeight = totalCubicFeet * 150;
// Calculate Bags
// 80lb bag yield approx 0.60 cubic feet
// 60lb bag yield approx 0.45 cubic feet
var bags80 = totalCubicFeet / 0.60;
var bags60 = totalCubicFeet / 0.45;
// Calculate Total Cost
var totalCost = totalCubicYards * cost;
// 4. Update UI
document.getElementById('resultArea').style.display = 'block';
document.getElementById('displayWaste').innerText = waste;
document.getElementById('resYards').innerText = totalCubicYards.toFixed(2);
document.getElementById('resFeet').innerText = totalCubicFeet.toFixed(2);
document.getElementById('resWeight').innerText = Math.ceil(totalWeight).toLocaleString();
document.getElementById('resCost').innerText = totalCost.toFixed(2);
document.getElementById('resBags80').innerText = Math.ceil(bags80);
document.getElementById('resBags60').innerText = Math.ceil(bags60);
}