Estimate cubic yards and pre-mix bags for your project
Estimated Materials Needed
*Includes a calculated 5% waste margin for spillage and uneven subgrade.
Total Volume (Cubic Yards):–
Total Volume (Cubic Feet):–
60lb Pre-Mix Bags:–
80lb Pre-Mix Bags:–
function calculateConcrete() {
var len = document.getElementById("slabLength").value;
var wid = document.getElementById("slabWidth").value;
var thk = document.getElementById("slabThickness").value;
var resultBox = document.getElementById("resultsArea");
// Validate Input
if (len === "" || wid === "" || thk === "") {
alert("Please fill in all fields (Length, Width, and Thickness).");
return;
}
var lengthFt = parseFloat(len);
var widthFt = parseFloat(wid);
var thickIn = parseFloat(thk);
if (isNaN(lengthFt) || isNaN(widthFt) || isNaN(thickIn) || lengthFt <= 0 || widthFt <= 0 || thickIn <= 0) {
alert("Please enter valid positive numbers.");
return;
}
// Calculation Logic
// 1. Convert thickness to feet
var thickFt = thickIn / 12;
// 2. Calculate Cubic Feet
var cubicFeet = lengthFt * widthFt * thickFt;
// 3. Add 5% Waste Margin
var totalCubicFeet = cubicFeet * 1.05;
// 4. Calculate Cubic Yards (27 cubic feet per yard)
var cubicYards = totalCubicFeet / 27;
// 5. Calculate Bags
// Standard Yield: 80lb bag ~= 0.60 cu ft, 60lb bag ~= 0.45 cu ft
var bags80 = Math.ceil(totalCubicFeet / 0.60);
var bags60 = Math.ceil(totalCubicFeet / 0.45);
// Display Results
document.getElementById("resYards").innerText = cubicYards.toFixed(2) + " yd³";
document.getElementById("resFeet").innerText = totalCubicFeet.toFixed(2) + " ft³";
document.getElementById("resBags60").innerText = bags60 + " Bags";
document.getElementById("resBags80").innerText = bags80 + " Bags";
// Show results div
resultBox.style.display = "block";
}
How to Calculate Concrete for Slabs and Patios
Accurately estimating the amount of concrete needed for a home improvement project is critical to avoiding shortages mid-pour or wasting money on excess materials. Whether you are pouring a patio, a driveway, or footings, the math relies on determining the total volume of the space in cubic yards.
The Concrete Formula
The basic formula to calculate concrete volume is: Length x Width x Thickness. However, because concrete is sold by the cubic yard (yd³) but dimensions are often measured in feet and inches, conversion is required.
Step 1: Convert the thickness from inches to feet by dividing by 12 (e.g., 4 inches ÷ 12 = 0.33 feet).
Step 2: Multiply Length (ft) x Width (ft) x Thickness (ft) to get Cubic Feet.
Step 3: Divide the total Cubic Feet by 27 to convert to Cubic Yards.
Pre-Mix Bags vs. Ready-Mix Truck
For smaller projects (typically under 2 cubic yards), using pre-mix bags (like Quikrete or Sakrete) is often more cost-effective.
80lb Bags: A standard 80lb bag yields approximately 0.60 cubic feet of cured concrete. You need about 45 bags to make one cubic yard.
60lb Bags: A 60lb bag yields approximately 0.45 cubic feet. You need about 60 bags to make one cubic yard.
For projects requiring more than 2-3 yards, ordering a ready-mix truck is usually preferred to ensure consistency and save physical labor.
Why Include a Waste Margin?
Our calculator automatically adds a 5% safety margin to the total. This accounts for:
Spillage during the pour.
Uneven subgrade or gravel base depth.
Settling of the concrete forms.
Running out of concrete during a pour can result in a "cold joint," which weakens the slab structurally and ruins the visual finish. It is always safer to order slightly more than the exact mathematical volume.