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Concrete Slab Calculator

Please enter valid positive numbers for length, width, and thickness.

function calculateConcrete() { // Get input values var length = parseFloat(document.getElementById("slabLength").value); var width = parseFloat(document.getElementById("slabWidth").value); var thickness = parseFloat(document.getElementById("slabThickness").value); var waste = parseFloat(document.getElementById("wasteFactor").value); // DOM elements for output var resultsDiv = document.getElementById("results"); var errorDiv = document.getElementById("errorMsg"); var outYards = document.getElementById("resYards"); var outFeet = document.getElementById("resFeet"); var outBags80 = document.getElementById("resBags80"); var outBags60 = document.getElementById("resBags60"); // Reset error state errorDiv.style.display = "none"; resultsDiv.style.display = "none"; // Validation if (isNaN(length) || length <= 0 || isNaN(width) || width <= 0 || isNaN(thickness) || thickness <= 0) { errorDiv.style.display = "block"; return; } // Handle empty waste input if (isNaN(waste) || waste < 0) { waste = 0; } // Calculations // 1. Convert thickness to feet var thicknessInFeet = thickness / 12; // 2. Calculate raw cubic feet var cubicFeet = length * width * thicknessInFeet; // 3. Add waste margin var totalCubicFeet = cubicFeet * (1 + (waste / 100)); // 4. Convert to cubic yards var cubicYards = totalCubicFeet / 27; // 5. Calculate bags // Standard yield: 80lb bag ≈ 0.60 cubic feet, 60lb bag ≈ 0.45 cubic feet var bags80 = Math.ceil(totalCubicFeet / 0.60); var bags60 = Math.ceil(totalCubicFeet / 0.45); // Output Results outYards.innerHTML = cubicYards.toFixed(2) + " yd³"; outFeet.innerHTML = totalCubicFeet.toFixed(2) + " ft³"; outBags80.innerHTML = bags80; outBags60.innerHTML = bags60; // Show results resultsDiv.style.display = "block"; }

How to Calculate Concrete for Slabs, Footings, and Patios

Planning a DIY concrete project requires precision. Ordering too little concrete leads to expensive delays and "cold joints" that weaken your structure, while ordering too much is a waste of money. Our Concrete Slab Calculator simplifies the math, helping you determine exactly how many cubic yards or pre-mix bags you need for your project.

Understanding the Concrete Formula

To calculate the volume of concrete required for a slab, you need to determine the volume in cubic feet and then convert that to cubic yards (which is how ready-mix trucks sell concrete). The basic formula is:

Length (ft) × Width (ft) × Thickness (ft) = Volume (cubic feet)

Since most people measure thickness in inches, you must first divide the inches by 12 to convert to feet. For example, a 4-inch slab is 0.33 feet thick.

Why Include a Waste Margin?

You will notice our calculator includes a "Waste Margin" input. Professional contractors rarely order the exact mathematical volume. Several factors contribute to volume loss during a pour:

• Uneven Subgrade: If the ground isn't perfectly flat, some areas of your slab might be 4.5 inches thick instead of 4 inches.
• Spillage: Concrete is heavy and messy; some will inevitably be lost during transport from the truck or mixer to the form.
• Form Bowing: Wooden forms can bow slightly outward under the weight of wet concrete, increasing the volume needed.

We recommend a standard safety margin of 5% to 10% to ensure you don't run short.

Truck vs. Pre-Mix Bags: Which Should You Choose?

Once you have your calculation, you need to decide how to buy your concrete.

1. Ready-Mix Truck

If your project requires more than 1 to 1.5 cubic yards, it is usually more cost-effective and physically easier to order a ready-mix truck. A standard concrete truck holds roughly 10 cubic yards. Keep in mind that many companies charge a "short load fee" for orders under a certain volume (often 4-6 yards).

2. Pre-Mix Bags (60lb or 80lb)

For smaller projects like setting fence posts, small walkways, or pads for AC units, buying bags from a home improvement store is ideal.

• 80lb Bag: Yields approximately 0.60 cubic feet of cured concrete.
• 60lb Bag: Yields approximately 0.45 cubic feet of cured concrete.

Our calculator instantly converts your total required volume into the specific number of bags needed so you can load up your truck with confidence.

Common Concrete Slab Thicknesses

Not sure how thick your slab should be? Here are the industry standards:

• 4 Inches: Standard for walkways, patios, and residential garage floors (for light vehicles).
• 5-6 Inches: Recommended for driveways holding heavy trucks, RVs, or heavy machinery.
• 6+ Inches: Heavy-duty industrial applications.

Tips for a Successful Pour

Calculating the right amount is step one. Ensure your subgrade is compacted firmly to prevent cracking later. Use rebar or wire mesh for tensile strength, and ensure your forms are level and securely staked. If you are mixing bags yourself, rent a mechanical mixer—mixing by hand in a wheelbarrow is exhausting for anything more than a few bags.