Estimate the volume and cost for patios, driveways, and sidewalks.
Material Type:
Please enter valid positive numbers for dimensions and price.
Total Area:0 sq ft
Volume (Cubic Yards):0 cu yd
80lb Bags Needed:0 bags
Estimated Weight:0 lbs
Estimated Total Cost:$0.00
How to Calculate Concrete Slab Costs
Whether you are pouring a new patio, a driveway extension, or a shed foundation, accurate calculation of concrete volume is critical to the success of your project. Ordering too little results in expensive "short load" fees or cold joints, while ordering too much is a waste of money and labor.
1. The Concrete Volume Formula
Concrete is measured in Cubic Yards. To calculate the requirements for a rectangular slab, you use the following formula:
Note: Since thickness is usually measured in inches, remember to divide the inches by 12 to get feet before multiplying.
2. Bags vs. Ready-Mix Truck
For small projects (under 1-2 cubic yards), pre-mixed bags (usually 60lb or 80lb) are cost-effective and manageable. An 80lb bag of concrete mix typically yields about 0.60 cubic feet of finished concrete.
For larger projects (driveways, large patios), ordering a Ready-Mix truck is usually more efficient. Trucks usually carry 9-10 cubic yards. Most ready-mix suppliers have a minimum order (often 1 yard) and may charge "short load fees" for small orders (under 4-5 yards).
3. Recommended Thickness
4 Inches: Standard for sidewalks, patios, and residential shed bases.
5-6 Inches: Recommended for driveways accommodating passenger cars.
6+ Inches: Heavy equipment or commercial aprons.
4. The Importance of the Waste Factor
No sub-base is perfectly flat. Excavation variations often mean you need slightly more concrete than the perfect mathematical volume. Professional contractors typically add a 5% to 10% safety margin (waste factor) to their orders to account for spillage, uneven ground, and form bowing.
function togglePriceLabel() {
var radios = document.getElementsByName('concType');
var priceLabel = document.getElementById('priceLabel');
var selected = 'bags';
for (var i = 0; i < radios.length; i++) {
if (radios[i].checked) {
selected = radios[i].value;
break;
}
}
if (selected === 'bags') {
priceLabel.innerText = "Price per 80lb Bag ($)";
document.getElementById('materialPrice').placeholder = "e.g. 5.50";
} else {
priceLabel.innerText = "Price per Cubic Yard ($)";
document.getElementById('materialPrice').placeholder = "e.g. 125.00";
}
}
function calculateConcrete() {
// Get Inputs
var len = parseFloat(document.getElementById('slabLength').value);
var wid = parseFloat(document.getElementById('slabWidth').value);
var thick = parseFloat(document.getElementById('slabThickness').value);
var waste = parseFloat(document.getElementById('wasteFactor').value);
var price = parseFloat(document.getElementById('materialPrice').value);
var errorDiv = document.getElementById('calcError');
var resultsDiv = document.getElementById('calcResults');
// Validation
if (isNaN(len) || isNaN(wid) || isNaN(thick) || len <= 0 || wid <= 0 || thick <= 0) {
errorDiv.style.display = 'block';
resultsDiv.style.display = 'none';
return;
} else {
errorDiv.style.display = 'none';
resultsDiv.style.display = 'block';
}
if (isNaN(price) || price < 0) price = 0;
if (isNaN(waste) || waste < 0) waste = 0;
// Logic
// 1. Calculate Square Footage
var sqFt = len * wid;
// 2. Calculate Cubic Feet (Thickness / 12 to convert inches to feet)
var cuFt = sqFt * (thick / 12);
// 3. Apply Waste Factor
var wasteMult = 1 + (waste / 100);
var totalCuFt = cuFt * wasteMult;
// 4. Convert to Cubic Yards
var cuYards = totalCuFt / 27;
// Determine Calculation Type (Bags vs Truck)
var radios = document.getElementsByName('concType');
var type = 'bags';
for (var i = 0; i < radios.length; i++) {
if (radios[i].checked) {
type = radios[i].value;
break;
}
}
var totalCost = 0;
var totalWeight = 0; // Concrete is approx 4000 lbs per yard, or 145-150 lbs per cu ft
var displayBags = 0;
var bagRow = document.getElementById('bagRow');
if (type === 'bags') {
// Logic for 80lb bags
// 80lb bag yields approx 0.6 cu ft
var yieldPerBag = 0.6;
var bagsNeeded = Math.ceil(totalCuFt / yieldPerBag);
totalCost = bagsNeeded * price;
totalWeight = bagsNeeded * 80;
displayBags = bagsNeeded;
// Show bag row
bagRow.style.display = 'flex';
document.getElementById('resBags').innerText = bagsNeeded + " bags";
} else {
// Logic for Truck
// Usually sold by the yard, often rounded up to nearest 0.25 or 0.5, but we will calculate raw cost
totalCost = cuYards * price;
totalWeight = cuYards * 4050; // Approx 4050 lbs per cu yard
// Hide bag row
bagRow.style.display = 'none';
}
// Display Results
document.getElementById('resArea').innerText = sqFt.toFixed(2) + " sq ft";
document.getElementById('resYards').innerText = cuYards.toFixed(2) + " cu yd";
document.getElementById('resWeight').innerText = Math.round(totalWeight).toLocaleString() + " lbs";
document.getElementById('resCost').innerText = "$" + totalCost.toFixed(2);
}