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Concrete Slab Calculator

Please enter valid positive numbers for Length, Width, and Thickness.
Total Volume Required
0 Cubic Yards
(0 Cubic Feet)
80lb Bags Needed
0
60lb Bags Needed
0
Estimated Bulk Cost
$0.00

How to Calculate Concrete for Your Project

Whether you are pouring a driveway, a patio, or a foundation slab, calculating the correct amount of concrete is crucial to the success of your project. Ordering too little can lead to structural weaknesses caused by 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.

The Concrete Volume Formula

Concrete is measured by volume. The standard unit of measurement for bulk concrete delivery is the Cubic Yard. For smaller DIY projects using pre-mix bags (like Quikrete or Sakrete), the volume is often calculated in cubic feet first.

The basic formula to calculate the volume of a rectangular slab is:

  • Volume (Cubic Feet) = Length (ft) × Width (ft) × Thickness (ft)
  • Volume (Cubic Yards) = Cubic Feet ÷ 27

Note: Since thickness is usually measured in inches, you must convert it to feet by dividing by 12 before multiplying.

Example Calculation

Let's say you want to pour a patio that is 12 feet long, 10 feet wide, and 4 inches thick.

  1. Convert thickness to feet: 4 inches ÷ 12 = 0.333 feet.
  2. Calculate cubic feet: 12 × 10 × 0.333 = 40 cubic feet.
  3. Convert to cubic yards: 40 ÷ 27 = 1.48 cubic yards.

Adding a standard 5-10% waste factor is highly recommended to account for spillage, uneven subgrades, and form settling.

How Many Bags of Concrete Do I Need?

If you aren't ordering a truck, you'll need to buy bags. Here are the standard yields for pre-mix concrete:

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

For the example above (40 cubic feet), you would need approximately 67 bags of 80lb concrete (40 ÷ 0.60).

Tips for a Successful Pour

1. Prepare the Subgrade: Ensure your ground is compacted and level. An uneven base can significantly increase the amount of concrete required.

2. Account for Waste: Professional contractors typically add 5% to 10% extra. For complex shapes or deeper edges, consider adding more.

3. Safety First: Wet concrete is heavy and caustic. Wear gloves, safety glasses, and long sleeves to prevent chemical burns.

function calculateConcrete() { // 1. Get input values using var var length = parseFloat(document.getElementById('slabLength').value); var width = parseFloat(document.getElementById('slabWidth').value); var thickness = parseFloat(document.getElementById('slabThickness').value); var wasteFactor = parseFloat(document.getElementById('wasteFactor').value); var pricePerYard = parseFloat(document.getElementById('priceYard').value); var pricePerBag = parseFloat(document.getElementById('priceBag').value); // 2. Validate inputs var errorDiv = document.getElementById('errorMessage'); var resultsDiv = document.getElementById('resultsArea'); if (isNaN(length) || length <= 0 || isNaN(width) || width <= 0 || isNaN(thickness) || thickness <= 0) { errorDiv.style.display = 'block'; resultsDiv.style.display = 'none'; return; } errorDiv.style.display = 'none'; resultsDiv.style.display = 'block'; // Handle waste factor default if (isNaN(wasteFactor) || wasteFactor 0) { var totalBulkCost = totalCubicYards * pricePerYard; document.getElementById('resBulkCost').innerText = '$' + totalBulkCost.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,'); bulkCard.style.display = 'block'; } else { bulkCard.style.display = 'none'; } // Bag Cost var bagCostDiv = document.getElementById('costBags80'); if (!isNaN(pricePerBag) && pricePerBag > 0) { var totalBagCost = bags80 * pricePerBag; bagCostDiv.innerText = 'Est. Cost: $' + totalBagCost.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,'); } else { bagCostDiv.innerText = "; } }

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