Planning a driveway, patio, or shed foundation? Accurate estimation is critical when working with concrete. Ordering too little results in expensive "short load" fees and cold joints, while ordering too much wastes budget. This Concrete Slab Calculator helps you determine exactly how many cubic yards or pre-mix bags you need, along with estimated material costs.
*Calculations include the selected waste margin to account for uneven subgrade and spillage.
function calculateConcrete() {
// Get Input Values
var len = parseFloat(document.getElementById('slabLength').value);
var wid = parseFloat(document.getElementById('slabWidth').value);
var thick = parseFloat(document.getElementById('slabThickness').value);
var waste = parseFloat(document.getElementById('wasteFactor').value);
var priceYard = parseFloat(document.getElementById('pricePerYard').value);
var priceBag = parseFloat(document.getElementById('pricePerBag').value);
// Validation
if (isNaN(len) || isNaN(wid) || isNaN(thick) || len <= 0 || wid <= 0 || thick <= 0) {
alert("Please enter valid positive numbers for Length, Width, and Thickness.");
return;
}
// Calculation Logic
// 1. Calculate Cubic Feet: L * W * (Thickness in Feet)
var thicknessInFeet = thick / 12;
var cubicFeetRaw = len * wid * thicknessInFeet;
// 2. Apply Waste Factor
var cubicFeetTotal = cubicFeetRaw * waste;
// 3. Convert to Cubic Yards (27 cubic feet = 1 cubic yard)
var cubicYardsTotal = cubicFeetTotal / 27;
// 4. Calculate Costs
var truckCost = cubicYardsTotal * priceYard;
// 5. Calculate Bags
// Standard yield: 80lb bag ~= 0.60 cubic feet, 60lb bag ~= 0.45 cubic feet
var bags80 = Math.ceil(cubicFeetTotal / 0.60);
var bags60 = Math.ceil(cubicFeetTotal / 0.45);
var diyCost = bags80 * priceBag;
// Display Results
document.getElementById('resYards').innerHTML = cubicYardsTotal.toFixed(2);
document.getElementById('resFeet').innerHTML = cubicFeetTotal.toFixed(2);
document.getElementById('resTruckCost').innerHTML = truckCost.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resBags80').innerHTML = bags80;
document.getElementById('resBags60').innerHTML = bags60;
document.getElementById('resDIYCost').innerHTML = diyCost.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Show Results Div
document.getElementById('resultsArea').style.display = 'block';
}
How to Calculate Concrete Requirements
Understanding the math behind your pour is essential for budgeting. Concrete is measured by volume, specifically in Cubic Yards. One cubic yard is equal to 27 cubic feet.
The Concrete Formula
The basic formula to calculate the concrete needed for a slab is:
(Length × Width × Thickness) ÷ 27 = Cubic Yards
Note: Ensure you convert the thickness from inches to feet (divide by 12) before multiplying by length and width.
Thickness Guide
4 Inches: Standard for sidewalks, patios, and residential garage floors.
5 Inches: Recommended for driveways supporting mid-sized vehicles.
6 Inches: Heavy-duty use, RV pads, or commercial aprons.
Ready-Mix Truck vs. Bagged Concrete
When should you order a truck versus buying bags from the hardware store?
Use Bags (DIY): For projects requiring less than 1 cubic yard (approx. 45-50 bags of 80lb mix). Ideal for setting fence posts, small landings, or repairs.
Order a Truck: For any project over 1 yard. The physical labor of mixing 50+ bags by hand is exhausting, and getting a consistent cure is difficult.
Don't Forget the Waste Factor!
Never order the exact mathematical amount. Subgrades are rarely perfectly flat, and forms may bow slightly under pressure. We recommend adding 10% extra to your order to account for spillage, uneven ground, and waste. It is much cheaper to discard a small amount of leftover concrete than to pay a "short load" fee for a second delivery of 0.5 yards.