How to Calculate Concrete for Slabs, Patios, and Driveways
Whether you are planning to pour a new driveway, a backyard patio, or a foundation for a shed, accuracy is crucial. Ordering too little concrete results in expensive delays and "cold joints," while ordering too much is a waste of money. Our Concrete Slab Calculator helps you determine exactly how many cubic yards or pre-mixed bags you need for your project.
The Basic Formula
Calculating concrete volume relies on simple geometry, but converting the units correctly is where most mistakes happen. The formula for a rectangular slab is:
Volume = Length × Width × Thickness
However, since concrete is sold by the Cubic Yard, but measurements are usually taken in feet and inches, you must convert everything to a common unit.
Step 1: Convert thickness from inches to feet (Divide by 12).
Step 2: Multiply Length (ft) × Width (ft) × Thickness (ft) to get Cubic Feet.
Step 3: Divide Cubic Feet by 27 (since there are 27 cubic feet in 1 cubic yard).
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 cars.
5-6 Inches: Recommended for driveways that handle heavier vehicles (trucks, RVs) or shed foundations.
8+ Inches: Heavy industrial use or commercial loading docks.
Pre-Mix Bags vs. Ready-Mix Truck
Should you buy bags from the hardware store or order a truck? Use the "Cubic Yards" result from the calculator to decide:
Under 1 Yard: It is usually more economical and manageable to use pre-mixed bags (60lb or 80lb) and rent a small mixer.
Over 1 Yard: The labor required to mix bags becomes overwhelming. Ordering a ready-mix truck is standard practice for volumes exceeding 1 to 1.5 cubic yards.
Understanding Yield
If you opt for bags, here is the typical yield:
An 80lb bag yields approximately 0.60 cubic feet.
A 60lb bag yields approximately 0.45 cubic feet.
Our calculator automatically performs this division for you, rounding up to the nearest whole bag to ensure you don't run short.
Why Add a Waste Margin?
Professional contractors never order the exact mathematical amount. Why? Because the ground is rarely perfectly level. A dip of just half an inch across a driveway can require significantly more concrete. Spillage during the pour and material remaining inside the mixer or pump also account for loss.
We recommend a 5% to 10% safety margin to account for uneven subgrades and spillage.
function calculateConcrete() {
// 1. Get input values using var
var length = document.getElementById("slabLength").value;
var width = document.getElementById("slabWidth").value;
var thickInch = document.getElementById("slabThickness").value;
var wastePercent = document.getElementById("wasteFactor").value;
var price = document.getElementById("pricePerYard").value;
// 2. Validation
if (!length || !width || !thickInch) {
alert("Please fill in Length, Width, and Thickness to calculate.");
return;
}
var l = parseFloat(length);
var w = parseFloat(width);
var t = parseFloat(thickInch);
var waste = parseFloat(wastePercent);
var costPerYard = parseFloat(price);
if (l <= 0 || w <= 0 || t 0) {
totalCost = totalCubicYards * costPerYard;
showCost = true;
}
// 6. Display Results
document.getElementById("resYards").innerHTML = totalCubicYards.toFixed(2) + " Cubic Yards";
document.getElementById("resFeet").innerHTML = totalCubicFeet.toFixed(2) + " cu. ft.";
document.getElementById("resBags80″).innerHTML = bags80 + " bags";
document.getElementById("resBags60″).innerHTML = bags60 + " bags";
var costRow = document.getElementById("costRow");
if (showCost) {
document.getElementById("resCost").innerHTML = "$" + totalCost.toFixed(2);
costRow.style.display = "flex";
} else {
costRow.style.display = "none";
}
document.getElementById("resultsArea").style.display = "block";
}