*Calculations include a standard 10% waste margin to account for spillage, uneven subgrade, and form deflection.
How to Calculate Concrete Requirements
Planning a new patio, driveway, or walkway involves precise measurements to ensure you order the right amount of material. Concrete is sold by volume, typically in cubic yards for bulk delivery (ready-mix trucks) or cubic feet for bagged products found at home improvement stores. This calculator simplifies the math by converting your dimensions into the necessary purchase units.
The Concrete Volume Formula
To determine the volume of a rectangular concrete slab, use the following formula:
Since thickness is commonly measured in inches, you must divide the inch value by 12 to convert it to feet before multiplying. For example, a 4-inch slab is 0.33 feet thick (4 ÷ 12).
Why Do I Need a Safety Margin?
Experienced contractors never order the exact mathematical volume. It is industry standard to add a 5% to 10% safety margin (or "waste factor"). Our calculator automatically includes a 10% buffer to account for:
Subgrade Variations: The ground is rarely perfectly flat; dips require more concrete to fill.
Spillage: Material lost during wheelbarrow transport or chute pouring.
Form Deflection: Wooden forms may bow slightly outward under the weight of wet concrete, increasing the slab's volume.
Settlement: Concrete may compact slightly after pouring.
Bagged Concrete vs. Ready-Mix Truck
Deciding between buying bags or ordering a truck depends on the project size:
Under 1 Cubic Yard: It is usually more economical to buy pre-mix bags (60lb or 80lb) and mix them yourself. A typical 80lb bag yields approximately 0.6 cubic feet of cured concrete.
Over 1 Cubic Yard: Mixing over 40-50 bags by hand is labor-intensive and risks "cold joints" (where one section hardens before the next is poured). For these projects, ordering a ready-mix truck is recommended for consistency and ease.
Standard Thickness Guidelines
Choosing the right thickness is vital for the longevity of your slab:
4 Inches: Standard for walkways, patios, and residential driveways used by passenger cars.
5 to 6 Inches: Recommended for hot tub pads, RV pads, or driveways housing heavier trucks.
6+ Inches: Heavy-duty commercial aprons or foundations supporting structural loads.
function calculateConcreteVolume() {
// Retrieve input values using var
var len = parseFloat(document.getElementById("slabLength").value);
var wid = parseFloat(document.getElementById("slabWidth").value);
var dep = parseFloat(document.getElementById("slabDepth").value);
var qty = parseFloat(document.getElementById("slabQty").value);
// Basic validation to ensure numbers are entered
if (isNaN(len) || isNaN(wid) || isNaN(dep)) {
alert("Please enter valid numbers for Length, Width, and Thickness.");
return;
}
// Handle quantity defaults
if (isNaN(qty) || qty < 1) {
qty = 1;
}
// 1. Calculate Volume in Cubic Feet (unpadded)
// Convert depth from inches to feet by dividing by 12
var depthInFeet = dep / 12;
var cubicFeetRaw = len * wid * depthInFeet * qty;
// 2. Add 10% Waste Margin
var cubicFeetTotal = cubicFeetRaw * 1.10;
// 3. Convert to Cubic Yards (27 cubic feet = 1 cubic yard)
var cubicYardsTotal = cubicFeetTotal / 27;
// 4. Calculate Bags Required
// Yield estimates: 80lb bag ~= 0.60 cu ft, 60lb bag ~= 0.45 cu ft
var bags80 = cubicFeetTotal / 0.60;
var bags60 = cubicFeetTotal / 0.45;
// 5. Update HTML Results
document.getElementById("resYards").innerHTML = cubicYardsTotal.toFixed(2);
document.getElementById("resCuFt").innerHTML = cubicFeetTotal.toFixed(2);
document.getElementById("resBags80").innerHTML = Math.ceil(bags80); // Round up to nearest whole bag
document.getElementById("resBags60").innerHTML = Math.ceil(bags60); // Round up to nearest whole bag
// 6. Show the results container
document.getElementById("resultsArea").style.display = "block";
}