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 costBag = parseFloat(document.getElementById('pricePerBag').value);
var costYard = parseFloat(document.getElementById('pricePerYard').value);
// Validation
if (isNaN(len) || isNaN(wid) || isNaN(thick) || len <= 0 || wid <= 0 || thick 0) {
// Usually you can't buy partial yards from a truck, but for estimation we multiply directly
// Or assume minimum load. Here we just do raw math for estimation.
var cost = cubicYardsTotal * costYard;
totalCostYard = "$" + cost.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
}
var totalCostBags = "N/A";
if (!isNaN(costBag) && costBag > 0) {
var cost = bags80 * costBag;
totalCostBags = "$" + cost.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
}
// Display Results
document.getElementById('resVolumeYards').innerHTML = cubicYardsTotal.toFixed(2) + " cu. yards";
document.getElementById('resVolumeFeet').innerHTML = cubicFeetTotal.toFixed(2) + " cu. ft";
document.getElementById('resBags80').innerHTML = bags80 + " bags";
document.getElementById('resBags60').innerHTML = bags60 + " bags";
document.getElementById('resCostYard').innerHTML = totalCostYard;
document.getElementById('resCostBags').innerHTML = totalCostBags;
document.getElementById('csResult').style.display = 'block';
}
How to Calculate Concrete for Your Project
Whether you are pouring a new patio, setting a driveway, or building a foundation for a shed, accurately calculating the amount of concrete required is the first step to a successful project. Ordering too little can result in disastrous "cold joints" where the concrete sets before you finish, while ordering too much wastes money.
The Basic Formula
The standard method for calculating concrete volume is to determine the cubic footage of the slab and then convert that into cubic yards (for truck orders) or number of bags (for DIY mixing).
The formula is: Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Note: Since thickness is usually measured in inches, you must divide the inches by 12 to get the measurement in feet.
Cubic Yards vs. Pre-Mix Bags
Once you have your total cubic footage, you need to decide how you will purchase the concrete:
Ready Mix Truck (Cubic Yards): Best for large projects (over 2 cubic yards). To get cubic yards, divide your total cubic feet by 27.
Pre-Mix Bags (60lb or 80lb): Best for small walkways, post holes, or repairs.
An 80lb bag typically yields about 0.60 cubic feet.
A 60lb bag typically yields about 0.45 cubic feet.
Why Include a Waste Factor?
Professional contractors always include a "margin of error" or waste factor in their calculations. This accounts for:
Spillage during transport or pouring.
Uneven subgrade (the ground isn't perfectly flat).
Settling of the sub-base materials.
Slight variations in formwork dimensions.
A standard waste factor is 5% to 10%. If you are digging into uneven ground or have a complex shape, consider adding 10% to be safe.
Standard Slab Thicknesses
4 Inches: Standard for residential sidewalks, patios, and garage floors for passenger cars.
5-6 Inches: Recommended for driveways that handle heavier vehicles or RVs.
6+ Inches: Heavy-duty foundations or areas with poor soil conditions.