* Results include a recommended 10% safety margin for spillage and uneven subgrades.
How to Calculate Concrete for a Slab
Planning a patio, driveway, or walkway? Calculating the correct amount of concrete is crucial to the success of your project. Ordering too little can lead to cold joints and structural weaknesses, while ordering too much is a waste of money. Our Concrete Slab Calculator simplifies the math by determining the exact volume required in cubic yards and the number of pre-mix bags needed.
The Concrete Formula
To calculate the concrete volume for a rectangular slab, you need three dimensions: length, width, and thickness (or depth). Since concrete is sold by the cubic yard in the US, but measurements are often taken in feet and inches, conversion is necessary.
The basic formula is:
Convert thickness from inches to feet: Thickness (in) ÷ 12
Experienced contractors always order slightly more concrete than the exact mathematical volume. This is known as the "waste factor" or "margin of safety." Our calculator automatically includes a standard 10% buffer in the results. This accounts for:
Uneven subgrade (dips in the ground)
Spillage during transport and pouring
Formwork settling or bowing
Variations in slab thickness
Pre-Mix Bags vs. Ready-Mix Truck
When should you use bags, and when should you call a truck?
Pre-Mix Bags (60lb or 80lb): Best for small projects requiring less than 1 cubic yard (approx. 45-60 bags). Ideal for setting fence posts, small walkways, or equipment pads.
Ready-Mix Truck: Recommended for projects over 1 cubic yard. Mixing over 60 bags of concrete by hand or with a small mixer is physically exhausting and makes it difficult to achieve a consistent pour before the concrete begins to set.
Standard Slab Thicknesses
4 Inches: Standard for residential sidewalks, patios, and garage floors for passenger cars.
5-6 Inches: Recommended for driveways that hold heavier vehicles (SUVs, trucks) or areas with poor soil conditions.
6+ Inches: Heavy-duty industrial floors or commercial driveways.
function calculateConcrete() {
// Get input values
var length = parseFloat(document.getElementById('slabLength').value);
var width = parseFloat(document.getElementById('slabWidth').value);
var thickness = parseFloat(document.getElementById('slabThickness').value);
var quantity = parseFloat(document.getElementById('slabQuantity').value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thickness)) {
alert("Please enter valid numbers for Length, Width, and Thickness.");
return;
}
if (isNaN(quantity) || quantity < 1) {
quantity = 1;
}
// Calculation Logic
// 1. Convert thickness to feet
var thicknessInFeet = thickness / 12;
// 2. Calculate Cubic Feet per slab
var cubicFeetPerSlab = length * width * thicknessInFeet;
// 3. Total Cubic Feet
var totalCubicFeet = cubicFeetPerSlab * quantity;
// 4. Add 10% Waste Margin
var totalCubicFeetWithWaste = totalCubicFeet * 1.10;
// 5. Convert to Cubic Yards (1 Yard = 27 Cubic Feet)
var totalCubicYards = totalCubicFeetWithWaste / 27;
// 6. Calculate Bags
// 80lb bag yields approx 0.6 cubic feet.
// 60lb bag yields approx 0.45 cubic feet.
// More precise: 1 cubic yard weighs ~4000 lbs (varies by mix, but standard avg)
// 4000 lbs / 80 = 50 bags per yard (approx)
// 4000 lbs / 60 = 66.6 bags per yard (approx)
// Using Yield method is safer:
// 80lb bag = 0.022 cubic yards
// 60lb bag = 0.017 cubic yards
var bags80 = Math.ceil(totalCubicYards / 0.0222); // 0.60 cu ft yield
var bags60 = Math.ceil(totalCubicYards / 0.0167); // 0.45 cu ft yield
// Display Results
document.getElementById('resYards').innerText = totalCubicYards.toFixed(2) + " cu yd";
document.getElementById('resFeet').innerText = totalCubicFeetWithWaste.toFixed(2) + " cu ft";
document.getElementById('resBags80').innerText = bags80;
document.getElementById('resBags60').innerText = bags60;
// Show results container
document.getElementById('results').style.display = 'block';
}