Whether you are pouring a simple backyard patio, a driveway, or footings for a deck, accuracy is crucial when ordering concrete. Ordering too little results in expensive "short load" fees or cold joints in your slab, while ordering too much is a waste of money and labor.
The Concrete Formula
Concrete is measured by volume. To find the volume of a rectangular slab, the formula is:
Since slab thickness is usually measured in inches, you must first convert that dimension to feet by dividing by 12. For example, a 4-inch slab is 0.33 feet thick.
To convert Cubic Feet to Cubic Yards (the standard unit for ordering ready-mix trucks), divide the total cubic feet by 27.
Pro Tip: Always include a "Waste Margin" or safety factor. We recommend 5% for perfectly formed rectangles and up to 10% for irregular shapes or uneven subgrades. This calculator defaults to a 5% safety margin.
How Many Bags of Concrete Do I Need?
If your project is small (under 1-2 cubic yards), it is often more economical to use premixed bags (like Quikrete or Sakrete) rather than ordering a truck. Here are the standard yields:
80lb Bag: Yields approximately 0.60 cubic feet.
60lb Bag: Yields approximately 0.45 cubic feet.
40lb Bag: Yields approximately 0.30 cubic feet.
Our calculator determines the exact bag count based on the volume required, rounded up to the nearest whole bag to ensure you don't run out mid-pour.
Standard Slab Thicknesses
4 Inches: Standard for residential sidewalks, patios, and steps.
5-6 Inches: Recommended for driveways and garage floors that hold passenger vehicles.
6+ Inches: Heavy equipment pads or RV parking.
function calculateConcrete() {
// 1. Get input values by ID
var lengthVal = document.getElementById("slabLength").value;
var widthVal = document.getElementById("slabWidth").value;
var thickVal = document.getElementById("slabThickness").value;
var wasteVal = document.getElementById("wasteFactor").value;
// 2. Validate inputs
if (lengthVal === "" || widthVal === "" || thickVal === "") {
alert("Please fill in Length, Width, and Thickness fields.");
return;
}
var L = parseFloat(lengthVal);
var W = parseFloat(widthVal);
var T_inches = parseFloat(thickVal);
var wastePercent = parseFloat(wasteVal) || 0;
if (L <= 0 || W <= 0 || T_inches <= 0) {
alert("Please enter positive values for dimensions.");
return;
}
// 3. Perform Calculations
// Convert thickness to feet (Inches / 12)
var T_feet = T_inches / 12;
// Calculate Volume in Cubic Feet
var cubicFeetRaw = L * W * T_feet;
// Add Waste Factor
var wasteMultiplier = 1 + (wastePercent / 100);
var totalCubicFeet = cubicFeetRaw * wasteMultiplier;
// Convert to Cubic Yards (1 Yard = 27 Cubic Feet)
var totalCubicYards = totalCubicFeet / 27;
// Calculate Bags
// 80lb bag ~= 0.60 cubic feet
// 60lb bag ~= 0.45 cubic feet
var bags80 = Math.ceil(totalCubicFeet / 0.60);
var bags60 = Math.ceil(totalCubicFeet / 0.45);
// 4. Update Result Display
document.getElementById("resYards").innerHTML = totalCubicYards.toFixed(2);
document.getElementById("resFeet").innerHTML = totalCubicFeet.toFixed(2);
document.getElementById("resBags80").innerHTML = bags80;
document.getElementById("resBags60").innerHTML = bags60;
// Show result container
document.getElementById("resultContainer").style.display = "block";
}