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Concrete Slab Calculator

0% (Exact) 5% (Recommended) 10% (Rough Terrain)

Estimated Materials:

Cubic Yards:

0.00

Cubic Feet:

0.00

Pre-Mixed Bags Needed:

80lb Bags 0
60lb Bags 0

*Bag counts are rounded up to the nearest whole bag. Includes selected safety margin.

function calculateConcrete() { // Get Input Values using var var lengthEl = document.getElementById('conc_length'); var widthEl = document.getElementById('conc_width'); var thickEl = document.getElementById('conc_thick'); var wasteEl = document.getElementById('conc_waste'); var resultsEl = document.getElementById('conc_results'); var lengthVal = parseFloat(lengthEl.value); var widthVal = parseFloat(widthEl.value); var thickVal = parseFloat(thickEl.value); var wasteVal = parseFloat(wasteEl.value); // Validation if (isNaN(lengthVal) || isNaN(widthVal) || isNaN(thickVal) || lengthVal <= 0 || widthVal <= 0 || thickVal <= 0) { alert("Please enter valid positive numbers for Length, Width, and Thickness."); resultsEl.style.display = 'none'; return; } // Calculation Logic // 1. Convert Thickness from inches to feet var thickFeet = thickVal / 12; // 2. Calculate Base Volume in Cubic Feet var volCubicFeet = lengthVal * widthVal * thickFeet; // 3. Add Waste Margin var wasteMultiplier = 1 + (wasteVal / 100); var totalCubicFeet = volCubicFeet * wasteMultiplier; // 4. Convert to Cubic Yards (27 cubic feet in 1 cubic yard) var totalCubicYards = totalCubicFeet / 27; // 5. Calculate Bags // Standard yield: 80lb bag ~= 0.60 cubic feet // Standard yield: 60lb bag ~= 0.45 cubic feet var bags80 = Math.ceil(totalCubicFeet / 0.60); var bags60 = Math.ceil(totalCubicFeet / 0.45); // Display Results document.getElementById('res_yards').innerHTML = totalCubicYards.toFixed(2) + " yd³"; document.getElementById('res_feet').innerHTML = totalCubicFeet.toFixed(2) + " ft³"; document.getElementById('res_bags80').innerText = bags80; document.getElementById('res_bags60').innerText = bags60; // Show Result Div resultsEl.style.display = 'block'; }

How to Calculate Concrete for Slabs and Patios

Planning a new driveway, patio, or shed foundation requires precise measurements to ensure you order enough concrete without overspending. This Concrete Calculator helps you determine exactly how much premix or ready-mix concrete you need for your project.

Understanding the Concrete Formula

To calculate the volume of concrete required for a rectangular slab, you use the basic volume formula: Length × Width × Thickness. However, the challenge lies in the units. Length and width are typically measured in feet, while thickness is measured in inches. Our calculator handles this conversion automatically using the following logic:

  1. Convert Thickness: Divide the inches by 12 to get the thickness in feet.
  2. Calculate Cubic Feet: Multiply Length (ft) × Width (ft) × Thickness (ft).
  3. Convert to Cubic Yards: Divide the total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).

Why Include a Safety Margin?

Experienced contractors always order more concrete than the exact mathematical volume. This is known as the "waste factor" or "safety margin." We recommend adding 5% to 10% extra for several reasons:

  • Spillage: Some concrete is inevitably lost during the pour or mixing process.
  • Uneven Subgrade: If your ground isn't perfectly level, some areas of the slab may be slightly thicker than planned.
  • Settling: Aggregate can settle, reducing the final volume slightly.

Bagged Concrete vs. Ready-Mix Truck

For smaller projects (typically under 1 to 1.5 cubic yards), buying bagged concrete (60lb or 80lb bags) from a home improvement store is cost-effective. For projects requiring more than 2 cubic yards, ordering a ready-mix truck is usually cheaper and saves significant physical labor.

Yield Reference:

  • One 80lb bag yields approximately 0.60 cubic feet.
  • One 60lb bag yields approximately 0.45 cubic feet.
  • One Cubic Yard requires roughly 45 bags (80lb) or 60 bags (60lb).

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