Bag counts are estimates based on standard yield (approx 0.6 cu ft per 80lb bag). Always round up to the nearest whole bag.
How to Calculate Concrete for a Slab
Whether you are pouring a patio, a driveway, or a foundation for a shed, accurately calculating the amount of concrete needed is crucial to avoid running out mid-pour or overspending on waste. This Concrete Slab Calculator helps you determine the exact volume in cubic yards and cubic feet, as well as the estimated number of pre-mix bags required for smaller DIY projects.
The Concrete Formula
To calculate the volume of concrete required for a slab, you need to determine the volume of the space in cubic feet and then convert it to cubic yards, which is the standard unit for ordering ready-mix concrete.
The basic formula is: Length (ft) × Width (ft) × Thickness (ft) = Volume (cubic feet)
Since slab thickness is usually measured in inches, you must divide the thickness by 12 to convert it to feet before multiplying.
To get Cubic Yards: Volume (cubic feet) ÷ 27 = Volume (cubic yards)
Recommended Slab Thickness
4 Inches: Standard for residential sidewalks, patios, and garage floors (for passenger cars).
5-6 Inches: Recommended for driveways that hold heavier vehicles (trucks, RVs) or areas with poor soil conditions.
6+ Inches: Heavy-duty foundations and commercial applications.
Why Add a Waste Margin?
It is industry standard to order 5% to 10% more concrete than your exact mathematical calculation. This accounts for:
Spillage during the pour.
Uneven subgrade (ground) depth.
Settling of the formwork.
Concrete remaining in the pump or truck chute.
Bags vs. Ready-Mix Truck
If your project requires less than 1 cubic yard of concrete, it is often more economical to use pre-mix bags (60lb or 80lb) available at home improvement stores. For projects requiring more than 1-2 cubic yards, ordering a ready-mix truck is usually more cost-effective and saves significant manual labor involved in mixing.
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function calculateConcrete() {
// Get Inputs
var lengthStr = document.getElementById('slab_length').value;
var widthStr = document.getElementById('slab_width').value;
var thickStr = document.getElementById('slab_thickness').value;
var wasteStr = document.getElementById('waste_percent').value;
var priceStr = document.getElementById('price_per_yard').value;
// Validation
if (lengthStr === "" || widthStr === "" || thickStr === "") {
alert("Please enter Length, Width, and Thickness.");
return;
}
var length = parseFloat(lengthStr);
var width = parseFloat(widthStr);
var thickness = parseFloat(thickStr);
var waste = parseFloat(wasteStr);
var price = priceStr === "" ? 0 : parseFloat(priceStr);
if (length <= 0 || width <= 0 || thickness 0) {
document.getElementById('res_cost').innerText = "$" + totalCost.toFixed(2);
document.getElementById('cost_row').style.display = "flex";
} else {
document.getElementById('cost_row').style.display = "none";
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