*Calculations include selected safety margin. Bag counts are rounded up to the nearest whole bag.
How to Calculate Concrete for Slabs and Patios
Planning a new patio, driveway, or shed foundation requires precise calculation of materials. Estimating concrete volume incorrectly can lead to expensive mid-project shortages or wasteful spending on excess material. This guide explains exactly how to determine the cubic yardage required for your project and translates that into practical buying units like pre-mix bags.
The Concrete Formula Explained
Concrete is sold by volume, not weight, typically measured in Cubic Yards for truck deliveries or Cubic Feet for small bag projects. The core formula for a rectangular slab is:
Volume = Length × Width × Thickness
However, the challenge lies in the units. Length and width are usually measured in feet, while thickness is measured in inches. To get an accurate result, you must convert the thickness into feet before multiplying:
Step 1: Convert thickness to feet (Divide inches by 12). Example: 4 inches ÷ 12 = 0.33 feet.
Step 2: Multiply Length (ft) × Width (ft) × Thickness (ft) to get Cubic Feet.
Step 3: Divide Cubic Feet by 27 to get Cubic Yards.
Why Include a Safety Margin?
Professional contractors never order the exact mathematical amount of concrete. Real-world conditions affect volume:
Uneven Subgrade: If your dirt base is slightly lower in the middle, you will need more concrete to level it.
Spillage: Some concrete is always lost during the pour or remains in the mixer/wheelbarrow.
Form Flexing: Wooden forms may bow slightly outward under the weight of wet concrete, increasing the volume.
We recommend a 5% safety margin for flat, well-prepared sites, and 10% for uneven ground or complex shapes.
Pre-Mix Bags vs. Ready-Mix Truck
Once you have your calculation, you must decide how to buy the material:
Bagged Concrete (60lb or 80lb): Best for projects under 1 cubic yard (approx. 45-60 bags). Ideal for fence posts, small walkways, or repairs.
Ready-Mix Truck: Best for projects over 1 cubic yard. It is more consistent and saves back-breaking labor, but requires a minimum order volume (often 3 to 5 yards) and quick work once the truck arrives.
Standard Slab Thickness Guide
Choosing the right thickness is critical for durability:
4 Inches: Standard for residential sidewalks, patios, and garage floors (for passenger cars).
5-6 Inches: Recommended for driveways holding heavier trucks, RVs, or hot tubs.
12+ Inches: Typically reserved for structural footings rather than slabs.
function calculateConcrete() {
// 1. Get Inputs
var lenStr = document.getElementById('slabLength').value;
var widStr = document.getElementById('slabWidth').value;
var thkStr = document.getElementById('slabThickness').value;
var wasteStr = document.getElementById('wasteFactor').value;
// 2. Validate Inputs
if (lenStr === "" || widStr === "" || thkStr === "") {
alert("Please fill in Length, Width, and Thickness to calculate.");
return;
}
var length = parseFloat(lenStr);
var width = parseFloat(widStr);
var thicknessInches = parseFloat(thkStr);
var wastePercent = parseFloat(wasteStr);
if (length <= 0 || width <= 0 || thicknessInches <= 0) {
alert("Dimensions must be greater than zero.");
return;
}
// 3. Calculation Logic
// Convert thickness to feet
var thicknessFeet = thicknessInches / 12;
// Calculate Cubic Feet (Base)
var cubicFeetBase = length * width * thicknessFeet;
// Apply Waste Factor
var wasteMultiplier = 1 + (wastePercent / 100);
var totalCubicFeet = cubicFeetBase * wasteMultiplier;
// Calculate Cubic Yards
var totalCubicYards = totalCubicFeet / 27;
// Calculate Bags
// Standard yields:
// 80lb bag approx 0.60 cubic feet
// 60lb bag approx 0.45 cubic feet
// 50lb bag approx 0.375 cubic feet (less common)
var bags80 = Math.ceil(totalCubicFeet / 0.60);
var bags60 = Math.ceil(totalCubicFeet / 0.45);
// 4. Update DOM
document.getElementById('resYards').innerHTML = totalCubicYards.toFixed(2);
document.getElementById('resFeet').innerHTML = totalCubicFeet.toFixed(2);
document.getElementById('resBags80').innerHTML = bags80;
document.getElementById('resBags60').innerHTML = bags60;
// Show Results Area
document.getElementById('results').style.display = "block";
// Optional: Scroll to results
document.getElementById('results').scrollIntoView({behavior: "smooth"});
}