0% (Exact)
5% (Recommended for rectangles)
10% (Recommended for irregular shapes)
15% (Safety margin)
Calculation Results
Total Volume Required:0 Cubic Yards
Volume in Cubic Feet:0 cu ft
Pre-Mix Bags Needed:
80lb Bags (0.60 cu ft):0 Bags
60lb Bags (0.45 cu ft):0 Bags
How to Calculate Concrete for Slabs and Footings
Planning a patio, driveway, or shed foundation requires precise measurements to ensure you order enough concrete without excessive waste. This Concrete Calculator determines exactly how much material you need in cubic yards (for truck delivery) or pre-mix bags (for smaller DIY projects).
The Concrete Calculation Formula
To calculate the volume of concrete required for a slab, you must calculate the volume in cubic feet and then convert it to cubic yards. The basic formula is:
Step 1: Convert all dimensions to feet. Since thickness is usually measured in inches, divide the inches by 12.
Step 2: Multiply Length × Width × Thickness (in feet) to get Cubic Feet.
Step 3: Divide the Cubic Feet by 27 to get Cubic Yards.
Example: For a 10′ x 10′ patio that is 4 inches thick:
10 x 10 x (4/12) = 33.33 Cubic Feet 33.33 / 27 = 1.23 Cubic Yards
Estimating Pre-Mix Bags (80lb vs 60lb)
If you are mixing the concrete yourself using bags from a hardware store, you need to know the yield of each bag. This calculator uses standard yields for pre-mixed concrete:
80lb Bag: Yields approximately 0.60 cubic feet.
60lb Bag: Yields approximately 0.45 cubic feet.
To determine the number of bags needed, divide your total project cubic footage by the bag yield. Always round up to the nearest whole bag.
Why Include Wastage?
Professional contractors always include a "margin of error" or wastage percentage. Uneven subgrades, spillage, and form setting variances can absorb more concrete than the theoretical math predicts. We recommend adding 5-10% extra material to prevent running short in the middle of a pour.
Standard Slab Thicknesses
4 Inches: Standard for sidewalks, patios, and residential driveways (passenger cars).
5-6 Inches: Recommended for heavier vehicle driveways (RVs, trucks) or shed bases.
8-12 Inches: Typically used for load-bearing footings or commercial applications.
function calculateConcrete() {
// 1. Get input values by ID
var length = parseFloat(document.getElementById('slab_length').value);
var width = parseFloat(document.getElementById('slab_width').value);
var thicknessInches = parseFloat(document.getElementById('slab_thickness').value);
var quantity = parseFloat(document.getElementById('slab_quantity').value);
var wastePercent = parseFloat(document.getElementById('slab_waste').value);
// 2. Validate inputs
if (isNaN(length) || length <= 0 ||
isNaN(width) || width <= 0 ||
isNaN(thicknessInches) || thicknessInches <= 0 ||
isNaN(quantity) || quantity <= 0) {
alert("Please enter valid positive numbers for all dimensions.");
return;
}
// 3. Calculation Logic
// Convert thickness to feet
var thicknessFeet = thicknessInches / 12;
// Calculate base volume in Cubic Feet
var volumeCuFt = length * width * thicknessFeet * quantity;
// Apply Wastage
var totalVolumeCuFt = volumeCuFt * (1 + (wastePercent / 100));
// Convert to Cubic Yards (27 cubic feet in 1 cubic yard)
var totalYards = totalVolumeCuFt / 27;
// Calculate Bags Needed
// 80lb bag yield approx 0.60 cu ft
// 60lb bag yield approx 0.45 cu ft
var bags80 = totalVolumeCuFt / 0.60;
var bags60 = totalVolumeCuFt / 0.45;
// 4. Update Result Display
document.getElementById('res_cu_ft').innerHTML = totalVolumeCuFt.toFixed(2) + " cu ft";
document.getElementById('res_yards').innerHTML = totalYards.toFixed(2) + " Cubic Yards";
// Use Math.ceil for bags because you can't buy partial bags
document.getElementById('res_bags_80').innerHTML = Math.ceil(bags80) + " Bags";
document.getElementById('res_bags_60').innerHTML = Math.ceil(bags60) + " Bags";
// Show result container
document.getElementById('cc-result').style.display = 'block';
}