Enter your slab dimensions to calculate volume and bag requirements.
Project Totals (Including 5% Waste)
Total Volume (Cubic Yards):–
Total Volume (Cubic Feet):–
80lb Bags Needed (Premix):–
60lb Bags Needed (Premix):–
function calculateConcrete() {
var length = parseFloat(document.getElementById('slabLength').value);
var width = parseFloat(document.getElementById('slabWidth').value);
var thick = parseFloat(document.getElementById('slabThickness').value);
var resultBox = document.getElementById('ccResult');
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thick) || length <= 0 || width <= 0 || thick <= 0) {
alert("Please enter valid positive numbers for all dimensions.");
resultBox.style.display = "none";
return;
}
// Calculations
// 1. Calculate base volume in Cubic Feet
// Thickness is in inches, so divide by 12 to get feet
var volCubicFeet = length * width * (thick / 12);
// 2. Add 5% safety margin for spillage and uneven subgrade
var volWithWaste = volCubicFeet * 1.05;
// 3. Convert to Cubic Yards (27 cubic feet = 1 cubic yard)
var volCubicYards = volWithWaste / 27;
// 4. Calculate Bags
// Approx yield: 80lb bag = 0.60 cu ft, 60lb bag = 0.45 cu ft
var bags80 = Math.ceil(volWithWaste / 0.60);
var bags60 = Math.ceil(volWithWaste / 0.45);
// Display Results
document.getElementById('resYards').innerHTML = volCubicYards.toFixed(2) + " yd³";
document.getElementById('resFeet').innerHTML = volWithWaste.toFixed(2) + " ft³";
document.getElementById('resBags80').innerHTML = bags80 + " Bags";
document.getElementById('resBags60').innerHTML = bags60 + " Bags";
resultBox.style.display = "block";
}
How to Calculate Concrete for Slabs and Patios
Planning a new driveway, patio, or shed foundation requires accurate material estimation. Ordering too little concrete results in expensive delays and cold joints, while ordering too much is a waste of money. This Concrete Slab Calculator helps you determine exactly how much premix or ready-mix concrete you need for your project.
The Concrete Volume Formula
To calculate the concrete volume, you must treat your slab as a rectangular prism. The math involves three standard dimensions:
Length (L): The longest side of the project area in feet.
Width (W): The shorter side of the project area in feet.
Thickness (T): The depth of the slab in inches (commonly 4″ for walkways or 6″ for driveways).
The standard formula used by this tool is:
Volume (ft³) = Length × Width × (Thickness ÷ 12)
Cubic Yards vs. Premix Bags
Once you have the volume, you need to decide how to buy the concrete:
Ready-Mix Truck (Cubic Yards): For projects requiring more than 1.5 cubic yards of concrete, it is usually more cost-effective and physically easier to order a truck. Concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet.
Pre-Mix Bags (60lb or 80lb): For smaller DIY projects like setting posts or small pads, buying bags from a hardware store is best.
An 80lb bag typically yields approximately 0.60 cubic feet of cured concrete.
A 60lb bag typically yields approximately 0.45 cubic feet of cured concrete.
Why Add a Safety Margin?
Professional contractors always add a "safety margin" or "waste factor" to their calculations. This calculator automatically adds a 5% buffer to your total volume. This accounts for:
Spillage during mixing or pouring.
Uneven subgrade (the ground isn't perfectly flat).
Slight bowing in wooden forms.
It is always better to have one bag left over than to run out halfway through your pour!