*Calculations include a standard 10% safety margin for spillage and uneven subgrade.
function calculateConcrete() {
var length = parseFloat(document.getElementById('slabLength').value);
var width = parseFloat(document.getElementById('slabWidth').value);
var thickness = parseFloat(document.getElementById('slabThickness').value);
var costPerYard = parseFloat(document.getElementById('concreteCost').value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thickness) || length <= 0 || width <= 0 || thickness 0) {
document.getElementById('resCost').innerText = "$" + totalCost.toFixed(2);
} else {
document.getElementById('resCost').innerText = "N/A";
}
// Show result box
document.getElementById('calcResults').className = "results-box active";
}
How to Calculate Concrete for Your Slab
Planning a DIY patio, driveway, or shed foundation? Accurate concrete calculations are critical to project success. Ordering too little concrete can lead to cold joints and structural weaknesses, while ordering too much wastes money. This Concrete Slab Calculator helps you determine exactly how many cubic yards or premix bags you need.
The Concrete Formula
To calculate the concrete volume, you need to find the volume of the space in cubic feet and then convert it to cubic yards. The formula is:
Since slab thickness is usually measured in inches, divide the inches by 12 first. For example, a 4-inch slab is 0.33 feet thick.
Finally, divide the total cubic feet by 27 to get Cubic Yards, which is how ready-mix trucks measure their load.
Premix Bags vs. Ready-Mix Truck
Should you mix it yourself or call a truck? Here is a general rule of thumb:
Small Projects (Under 1 Yard): Use 60lb or 80lb bags of premix (like Quikrete or Sakrete). It is labor-intensive but cheaper for small areas like walkway pads or post holes.
Large Projects (Over 1 Yard): Order a ready-mix truck. One cubic yard of concrete weighs about 4,000 lbs. Trying to mix 45+ bags of concrete by hand is physically exhausting and difficult to finish before the concrete sets.
Why Add a Waste Margin?
Professional contractors always order 5% to 10% extra concrete. This accounts for:
Uneven subgrade (dips in the ground)
Spillage during the pour
Settling of the forms
Our calculator automatically provides a "Recommended Volume" that includes a 10% safety margin to ensure you don't run dry in the middle of your pour.
Standard Slab Thicknesses
4 Inches: Standard for walkways, patios, and residential driveways (passenger cars).
5-6 Inches: Recommended for heavy-duty driveways (trucks/RVs) or hot tub pads.
6+ Inches: Heavy commercial use or foundations requiring reinforcement rebar.