*Results include the specified waste margin. Bag counts are rounded up to the nearest whole bag. Calculation assumes standard concrete density of approx. 145-150 lbs/ft³.
function calculateConcrete() {
// Get input values
var len = parseFloat(document.getElementById('cc_length').value);
var wid = parseFloat(document.getElementById('cc_width').value);
var thick = parseFloat(document.getElementById('cc_thickness').value);
var waste = parseFloat(document.getElementById('cc_waste').value);
// Validation
if (isNaN(len) || isNaN(wid) || isNaN(thick)) {
alert("Please enter valid numbers for Length, Width, and Thickness.");
return;
}
if (isNaN(waste)) {
waste = 0;
}
// Calculations
// 1. Convert thickness to feet
var thickFt = thick / 12;
// 2. Calculate raw cubic feet
var cubicFtRaw = len * wid * thickFt;
// 3. Add waste margin
var totalCubicFt = cubicFtRaw * (1 + (waste / 100));
// 4. Convert to cubic yards (27 cubic feet = 1 cubic yard)
var cubicYards = totalCubicFt / 27;
// 5. Calculate bags
// Assumed density of cured concrete is ~145-150 lbs per cubic foot.
// We use 150lbs to be safe (typical for pre-mix).
var totalWeightLbs = totalCubicFt * 150;
var bags60 = Math.ceil(totalWeightLbs / 60);
var bags80 = Math.ceil(totalWeightLbs / 80);
// Display Results
document.getElementById('res_yards').innerHTML = cubicYards.toFixed(2) + " yd³";
document.getElementById('res_feet').innerHTML = totalCubicFt.toFixed(2) + " ft³";
document.getElementById('res_bags60').innerHTML = bags60 + " bags";
document.getElementById('res_bags80').innerHTML = bags80 + " bags";
// Show result div
document.getElementById('cc-results').style.display = 'block';
}
Concrete Slab Calculator: Estimate Bags & Yards
Whether you are pouring a new patio, a driveway extension, or a simple shed base, accurate material estimation is crucial. Our Concrete Slab Calculator helps you determine exactly how much concrete you need in both cubic yards (for ready-mix trucks) and premix bags (for DIY projects).
How to Calculate Concrete Volume
The formula for calculating the volume of a concrete slab is relatively simple geometry, but it requires consistent units. The basic formula is:
Volume = Length × Width × Thickness
However, since length and width are usually measured in feet and thickness in inches, you must convert the thickness to feet first:
Divide the thickness (inches) by 12 to get feet.
Multiply Length (ft) × Width (ft) × Thickness (ft) to get Cubic Feet.
Divide Cubic Feet by 27 to get Cubic Yards.
Example Calculation
Let's say you are building a 10ft x 10ft patio that is 4 inches thick.
For smaller projects where ordering a truck isn't feasible, you will likely buy premixed bags from a hardware store. Common sizes are 60lb and 80lb bags.
80lb Bag Yield: Approximately 0.60 cubic feet of cured concrete.
60lb Bag Yield: Approximately 0.45 cubic feet of cured concrete.
Using the example above (33.33 cubic feet required), you would calculate:
33.33 ft³ ÷ 0.60 ft³/bag = 55.5 bags (Round up to 56 bags of 80lb mix).
Why Include a Waste Factor?
Professional contractors always include a "waste margin" or "contingency" in their calculations. This accounts for:
Spillage during mixing or pouring.
Uneven subgrade (ground) depth (some spots might be 4.5 inches deep instead of 4).
Concrete remaining stuck in the mixer or wheelbarrow.
We recommend a 5% to 10% waste margin for most flatwork projects to ensure you don't run out of material mid-pour.
Common Concrete Thickness Guidelines
4 Inches: Standard for sidewalks, patios, and residential driveways (passenger cars).
5-6 Inches: Required for heavier driveways (trucks/SUVs) or garage floors.
6+ Inches: Heavy-duty foundations or commercial aprons.