Please enter valid positive numbers for all fields.
Total Volume (Cubic Yards):0
Total Volume (Cubic Feet):0
80lb Premix Bags Needed:0
60lb Premix Bags Needed:0
*Calculations include selected safety margin. Bag counts rounded up to next whole bag.
How to Calculate Concrete for Slabs
Planning a patio, driveway, or foundation pour? Determining the correct amount of concrete is critical to the success of your project. Ordering too little results in expensive delays and "cold joints" in your slab, while ordering too much is a waste of money.
The Concrete Formula
To calculate the concrete volume required for a rectangular slab, you need to determine the volume in cubic feet and then convert it to cubic yards (the standard unit for ordering from ready-mix trucks). The formula is:
Step 1: Multiply Length (ft) × Width (ft) to get the area in square feet.
Step 2: Convert Thickness (in) to feet by dividing by 12.
Step 3: Multiply Area (sq ft) × Thickness (ft) to get Volume in Cubic Feet.
Step 4: Divide Cubic Feet by 27 to get Cubic Yards.
Premix Bags vs. Ready-Mix Truck
When should you use bags versus ordering a truck?
Premix Bags (60lb or 80lb): Best for small projects requiring less than 1 cubic yard of concrete (e.g., setting fence posts, small walkways, or pads). It is labor-intensive to mix by hand or with a small rental mixer.
Ready-Mix Truck: Recommended for any project over 1 cubic yard. The concrete arrives pre-mixed and consistent, saving massive amounts of labor. Be aware that most trucks have a minimum order quantity (often 1 yard + a short load fee).
Why Add a Safety Margin?
Our calculator defaults to a 5% safety margin, often called a "waste factor." This accounts for:
Uneven subgrade depth (holes or dips in the dirt).
Spillage during the pour.
Concrete remaining in the mixer or wheelbarrow.
Form bowing under the weight of wet concrete.
For uneven ground or complex shapes, consider increasing your safety margin to 10%.
Common Slab Thicknesses
4 Inches: Standard for walkways, patios, and residential floors.
5-6 Inches: Driveways (passenger vehicles) and light garage floors.
6+ Inches: Heavy equipment pads, RV parking, or structural foundations.
function calculateConcrete() {
// Get input values
var length = document.getElementById('slabLength').value;
var width = document.getElementById('slabWidth').value;
var thickness = document.getElementById('slabThickness').value;
var waste = document.getElementById('wasteFactor').value;
var errorMsg = document.getElementById('errorMsg');
var resultsDiv = document.getElementById('calc-results');
// Validation
if (length === "" || width === "" || thickness === "" || length <= 0 || width <= 0 || thickness <= 0) {
errorMsg.style.display = "block";
resultsDiv.style.display = "none";
return;
} else {
errorMsg.style.display = "none";
}
// Parse values to floats
var l = parseFloat(length);
var w = parseFloat(width);
var t_inches = parseFloat(thickness);
var w_factor = parseFloat(waste);
// Logic: Convert thickness to feet
var t_feet = t_inches / 12;
// Calculate Cubic Feet
var cubicFeetRaw = l * w * t_feet;
var cubicFeetTotal = cubicFeetRaw * w_factor;
// Calculate Cubic Yards (27 cubic feet in 1 cubic yard)
var cubicYardsTotal = cubicFeetTotal / 27;
// Calculate Bags
// Standard yield: 80lb bag ~= 0.60 cu ft, 60lb bag ~= 0.45 cu ft
var bags80 = Math.ceil(cubicFeetTotal / 0.60);
var bags60 = Math.ceil(cubicFeetTotal / 0.45);
// Update Display
document.getElementById('resYards').innerHTML = cubicYardsTotal.toFixed(2);
document.getElementById('resFeet').innerHTML = cubicFeetTotal.toFixed(2);
document.getElementById('resBags80').innerHTML = bags80;
document.getElementById('resBags60').innerHTML = bags60;
// Show Results
resultsDiv.style.display = "block";
}