Please enter valid dimensions (Length, Width, and Thickness).
Total Volume Needed
0 Cubic Yards
Volume in Cubic Feet:0 ft³
80lb Bags Needed
0
60lb Bags Needed
0
Est. Cost (Truck Delivery):$0.00
Est. Cost (80lb Pre-mix):$0.00
Comprehensive Guide to Calculating Concrete Slab Costs
Planning a patio, driveway, or shed foundation requires accurate estimation of concrete materials. Ordering too little concrete can result in "cold joints" and structural weaknesses, while ordering too much is a waste of money. This guide explains how to calculate cubic yardage, decide between pre-mix bags vs. ready-mix trucks, and estimate your total project costs.
The Concrete Calculation Formula
Concrete is measured by volume in Cubic Yards. To determine how much you need, you must calculate the volume of your slab in cubic feet and convert it.
Formula: (Length in feet × Width in feet × Thickness in feet) ÷ 27 = Cubic Yards
Note that thickness is usually measured in inches, so you must divide the inches by 12 to get the thickness in feet before multiplying.
Pre-Mix Bags vs. Ready-Mix Truck
Once you know your volume, you need to decide how to source the concrete:
Pre-Mix Bags (60lb or 80lb): Best for small projects under 1 cubic yard (e.g., small walkways, post holes, tiny pads). It requires manual mixing and significant physical labor.
Ready-Mix Truck: Best for projects over 1 cubic yard (e.g., driveways, large patios, garage floors). The concrete arrives pre-mixed and is poured directly via a chute. Note that trucks often have "short load fees" for orders under 3-4 yards.
Important Factors Affecting Cost
When estimating the price of your concrete slab, consider these variables:
Waste Factor: Always add 5-10% extra to your calculation to account for spillage, uneven sub-grade depths, and form bowing. Our calculator allows you to select a safety margin.
Sub-base Preparation: A proper slab requires 4-6 inches of compacted gravel underneath. This material cost is separate from the concrete itself.
Reinforcement: Rebar or wire mesh adds strength and crack resistance but increases material costs.
Finishing Tools: Don't forget the cost of rental equipment like bull floats, screeds, or mixers if you are doing it yourself.
Frequently Asked Questions
How many 80lb bags of concrete make a cubic yard?
It typically takes about 45 bags of 80lb pre-mix concrete to make one cubic yard. If using 60lb bags, you will need approximately 60 bags.
What is the standard thickness for a concrete slab?
For standard residential patios and walkways, 4 inches is the standard thickness. For driveways or slabs supporting heavy machinery, 5 to 6 inches is recommended.
Do I need rebar in my 4-inch slab?
While not strictly required for all foot traffic areas, adding wire mesh or rebar significantly reduces the likelihood of cracks separating and shifting over time, especially in climates with freeze-thaw cycles.
function calculateConcrete() {
// 1. Get Input Values
var lenInput = document.getElementById('slabLength');
var widInput = document.getElementById('slabWidth');
var thickInput = document.getElementById('slabThickness');
var wasteInput = document.getElementById('wasteFactor');
var priceYardInput = document.getElementById('pricePerYard');
var priceBagInput = document.getElementById('pricePerBag');
var errorBox = document.getElementById('errorBox');
var resultBox = document.getElementById('resultBox');
// 2. Parse values
var length = parseFloat(lenInput.value);
var width = parseFloat(widInput.value);
var thicknessInches = parseFloat(thickInput.value);
var wastePercent = parseFloat(wasteInput.value);
var pricePerYard = parseFloat(priceYardInput.value);
var pricePerBag = parseFloat(priceBagInput.value);
// 3. Validation
if (isNaN(length) || isNaN(width) || isNaN(thicknessInches) || length <= 0 || width <= 0 || thicknessInches 0) {
totalCostYard = totalCubicYards * pricePerYard;
document.getElementById('resCostYard').innerText = '$' + totalCostYard.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('costRowYard').style.display = 'flex';
} else {
document.getElementById('costRowYard').style.display = 'none';
}
if (!isNaN(pricePerBag) && pricePerBag > 0) {
totalCostBag = bags80 * pricePerBag; // Assuming 80lb bags for cost calc
document.getElementById('resCostBag').innerText = '$' + totalCostBag.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('costRowBag').style.display = 'flex';
} else {
document.getElementById('costRowBag').style.display = 'none';
}
}