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Concrete Slab & Patio Calculator

function calculateConcrete() { // Get Input Values var len = document.getElementById('slabLength').value; var wid = document.getElementById('slabWidth').value; var thick = document.getElementById('slabThickness').value; var price = document.getElementById('costPerYard').value; var waste = document.getElementById('wasteFactor').value; // Validate Inputs if (len === "" || wid === "" || thick === "" || price === "") { alert("Please fill in all dimensions and price fields."); return; } // Convert strings to floats var l = parseFloat(len); var w = parseFloat(wid); var t = parseFloat(thick); var p = parseFloat(price); var wst = parseFloat(waste); if (isNaN(l) || isNaN(w) || isNaN(t) || isNaN(p)) { alert("Please enter valid numbers."); return; } if (isNaN(wst)) { wst = 0; } // 1. Calculate Cubic Feet (L * W * (Thickness in feet)) var thicknessInFeet = t / 12; var cubicFeet = l * w * thicknessInFeet; // 2. Add Wastage var wasteMultiplier = 1 + (wst / 100); var totalCubicFeet = cubicFeet * wasteMultiplier; // 3. Convert to Cubic Yards (1 Yard = 27 Cubic Feet) var totalCubicYards = totalCubicFeet / 27; // 4. Calculate Bags (Approx 0.6 cubic feet yield per 80lb bag) var yieldPer80lb = 0.6; var bagsNeeded = Math.ceil(totalCubicFeet / yieldPer80lb); // 5. Calculate Cost var totalCost = totalCubicYards * p; // Display Results document.getElementById('res-yards').innerHTML = totalCubicYards.toFixed(2) + " Cubic Yards"; document.getElementById('res-feet').innerHTML = totalCubicFeet.toFixed(2) + " Cubic Feet"; document.getElementById('res-bags').innerHTML = bagsNeeded + " Bags (80lb)"; document.getElementById('res-cost').innerHTML = "$" + totalCost.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2}); // Show Result Div document.getElementById('result-area').style.display = "block"; }

How to Calculate Concrete for Slabs and Patios

Planning a new driveway, patio, or slab foundation requires precise measurements to ensure you order enough material without overspending. Concrete is typically sold by the cubic yard (if ordering a truck) or by the bag (for smaller DIY projects).

The Concrete Volume Formula

To determine the volume of concrete needed for a rectangular slab, you use the following geometric formula:

Volume = Length × Width × Thickness

However, since length and width are usually measured in feet and thickness in inches, you must convert the thickness to feet before multiplying:

• Step 1: Measure Length and Width in feet.
• Step 2: Measure Thickness in inches and divide by 12 to get feet (e.g., 4 inches ÷ 12 = 0.33 feet).
• Step 3: Multiply Length × Width × Height (in feet) to get Cubic Feet.
• Step 4: Divide Cubic Feet by 27 to get Cubic Yards.

Typical Concrete Thickness Standards

The thickness of your slab depends heavily on its intended use:

• 4 Inches: Standard for residential sidewalks, patios, and garage floors for passenger cars.
• 5-6 Inches: Recommended for driveways that hold heavier vehicles (SUVs, trucks) or high-traffic areas.
• 6+ Inches: Heavy-duty industrial slabs or areas supporting RVs and heavy machinery.

Accounting for Wastage and Spillage

It is standard practice in the construction industry to order more concrete than the exact mathematical volume. Uneven subgrades, spilling during pouring, and spreading discrepancies can reduce your yield.

We recommend adding a 5% to 10% safety margin (wastage factor) to your order. If you are pouring 5 cubic yards, ordering 5.5 yards ensures you won't run short in the middle of the job.

Ready-Mix Truck vs. Bagged Concrete

When should you order a truck versus buying bags from the hardware store?

• Bagged Concrete: Best for small projects under 1 cubic yard (approx. 45-50 bags of 80lb mix). It is labor-intensive to mix by hand.
• Ready-Mix Truck: Best for projects over 1 cubic yard. It is pumped or poured directly into your forms, saving time and ensuring a consistent mix.