Enter your dimensions to calculate cubic yards and premix bags needed.
0% (Exact)
5% (Recommended)
10% (Safe)
Estimated Materials Needed
Total Volume:0 Cubic Yards
Cubic Feet:0 ft³
Premix Bag Options (Buy ONE option):
80lb Bags (Quikrete/Sakrete):0 Bags
60lb Bags:0 Bags
function calculateConcrete() {
// Get input values
var length = parseFloat(document.getElementById('conc_length').value);
var width = parseFloat(document.getElementById('conc_width').value);
var thickness = parseFloat(document.getElementById('conc_thickness').value);
var wasteFactor = parseFloat(document.getElementById('conc_waste').value);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(thickness) || length <= 0 || width <= 0 || thickness <= 0) {
alert("Please enter valid positive numbers for all dimensions.");
return;
}
// Calculations
// 1. Calculate cubic feet: Length (ft) * Width (ft) * Thickness (ft)
// Thickness is in inches, so divide by 12
var cubicFeetRaw = length * width * (thickness / 12);
// Apply waste margin
var cubicFeetTotal = cubicFeetRaw * wasteFactor;
// 2. Convert to Cubic Yards (1 Yard = 27 Cubic Feet)
var cubicYards = cubicFeetTotal / 27;
// 3. Calculate Bags
// A standard 80lb bag yields approx 0.60 cubic feet
// A standard 60lb bag yields approx 0.45 cubic feet
var bags80 = Math.ceil(cubicFeetTotal / 0.60);
var bags60 = Math.ceil(cubicFeetTotal / 0.45);
// Display Results
document.getElementById('res_cu_yards').innerHTML = cubicYards.toFixed(2) + " Cubic Yards";
document.getElementById('res_cu_feet').innerHTML = cubicFeetTotal.toFixed(2) + " ft³";
document.getElementById('res_bags_80').innerHTML = bags80 + " Bags";
document.getElementById('res_bags_60').innerHTML = bags60 + " Bags";
// Show result box
document.getElementById('conc_results').style.display = 'block';
}
Concrete Slab Calculation Guide
Planning a concrete project requires precise volume calculations to ensure you order enough material without overspending. Whether you are pouring a patio, a driveway, or a shed foundation, understanding how to calculate the cubic yardage and bag count is the first step to a successful project.
How is Concrete Measured?
Concrete is sold by volume, not just weight. The two primary units of measurement you need to know are:
Cubic Yards: If you are ordering a ready-mix truck, you will order in "yards". One cubic yard is equal to 27 cubic feet.
Premix Bags: For smaller DIY projects, concrete comes in 60lb or 80lb bags. These are measured by the cubic foot yield.
The Formula
To calculate the concrete needed for a slab, use the following formula:
(Length ft × Width ft × Thickness in) ÷ 12 = Cubic Feet
Once you have the cubic feet, divide by 27 to get cubic yards.
Recommended Slab Thickness
The thickness of your slab depends heavily on its intended use. Using the wrong thickness can lead to cracking under load.
4 Inches: Standard for walkways, patios, and residential driveways (passenger cars only).
5-6 Inches: Recommended for driveways that accommodate heavier trucks, RVs, or hot tubs.
A common rule of thumb in construction is the "1 Yard Rule".
Under 1 Cubic Yard: It is usually more cost-effective and manageable to buy bags (60lb or 80lb) and mix them yourself or use a small electric mixer. Note that 1 yard of concrete requires roughly 45 bags (80lb), which is physically demanding.
Over 1 Cubic Yard: It is highly recommended to order a ready-mix truck. The consistency will be better, and it saves immense physical labor. However, trucks often have "short load fees" for orders under 3-4 yards.
Why Include a Safety Margin?
This calculator includes a "Safety Margin" option (default 10%). Concrete projects rarely have perfectly level subgrades. A dip in the dirt of just half an inch across a driveway can increase your volume needs significantly. Spillage during the pour is also common. It is always cheaper to buy 10% extra concrete than to pay for a second delivery or run to the store while your wet edge is drying.