Please enter valid positive numbers for all dimensions.
Total Volume
0
Cubic Yards
60lb Premix Bags
0
Bags Needed
80lb Premix Bags
0
Bags Needed
*Results include the selected waste margin to account for spillage and uneven ground.
function calculateConcrete() {
// 1. Get input values
var length = document.getElementById('concLength').value;
var width = document.getElementById('concWidth').value;
var thickness = document.getElementById('concThick').value;
var wastePercent = document.getElementById('concWaste').value;
var resultsDiv = document.getElementById('concResults');
var errorDiv = document.getElementById('concError');
// 2. Validate Inputs
if (length === "" || width === "" || thickness === "" || isNaN(length) || isNaN(width) || isNaN(thickness)) {
errorDiv.style.display = "block";
resultsDiv.style.display = "none";
return;
}
// Parse floats
var l = parseFloat(length);
var w = parseFloat(width);
var t = parseFloat(thickness);
var waste = parseFloat(wastePercent);
// Check for non-positive numbers
if (l <= 0 || w <= 0 || t <= 0) {
errorDiv.style.display = "block";
resultsDiv.style.display = "none";
return;
}
// 3. Perform Calculations
// Hide error if valid
errorDiv.style.display = "none";
// Calculate Cubic Feet: (L * W * (T / 12))
var cubicFeet = l * w * (t / 12);
// Add Waste Margin
var totalCubicFeet = cubicFeet * (1 + (waste / 100));
// Calculate Cubic Yards: Cubic Feet / 27
var cubicYards = totalCubicFeet / 27;
// Calculate Bags
// 60lb bag yields approx 0.45 cubic feet
var bags60 = totalCubicFeet / 0.45;
// 80lb bag yields approx 0.60 cubic feet
var bags80 = totalCubicFeet / 0.60;
// 4. Update UI
// Round yards to 2 decimals
document.getElementById('resYards').innerHTML = cubicYards.toFixed(2);
// Round bags up to nearest whole integer
document.getElementById('resBags60').innerHTML = Math.ceil(bags60);
document.getElementById('resBags80').innerHTML = Math.ceil(bags80);
// Show results
resultsDiv.style.display = "block";
}
How to Calculate Concrete for Slabs and Footings
Planning a new patio, driveway, or walkway? Determining the exact amount of concrete required is the most critical step in the budgeting and execution process. Ordering too little can result in catastrophic "cold joints" where the pour is interrupted, while ordering too much wastes money and creates disposal issues. This guide helps you navigate the specific math behind concrete volume estimation.
The Concrete Volume Formula
Concrete is measured by volume, specifically in Cubic Yards (often just called "yards"). To find this, you must think in three dimensions: Length, Width, and Thickness.
The standard formula used by contractors is:
(Length(ft) × Width(ft) × Thickness(in) / 12) / 27 = Cubic Yards
Why divide by 12? This converts your thickness from inches into feet so all units match.
Why divide by 27? There are 27 cubic feet in one cubic yard (3ft x 3ft x 3ft).
Standard Slab Thicknesses
Choosing the right thickness affects your calculation significantly:
4 Inches: The industry standard for residential sidewalks, patios, and garage floors for passenger cars.
5-6 Inches: Recommended for driveways that hold heavier SUVs, trucks, or receive frequent traffic.
8+ Inches: Heavy-duty commercial applications or areas supporting heavy machinery.
Premix Bags vs. Ready-Mix Truck
Once you have your calculated volume, you need to decide how to buy the concrete.
For projects under 1.5 Cubic Yards: It is usually cheaper and easier to mix bagged concrete yourself. You can buy 60lb or 80lb bags at local hardware stores.
An 80lb bag yields approximately 0.6 cubic feet.
A 60lb bag yields approximately 0.45 cubic feet.
For projects over 2 Cubic Yards: Ordering a "Ready-Mix" truck is highly recommended. Mixing by hand becomes labor-prohibitive at this volume. Note that many concrete plants have a "short load" fee for orders under 4-5 yards.
Why Include a Waste Margin?
In our calculator, we include a default margin of 10%. Why? Excavated ground is rarely perfectly flat. If your subgrade dips down by just half an inch across a 20ft driveway, you will run out of concrete before finishing. Additionally, some concrete is always lost in the mixer or spilt during transport. Professional contractors typically order 5% to 10% extra to ensure the job can be completed in one pour.