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Planning a driveway, patio, or slab? Use our Concrete Calculator to determine exactly how many cubic yards of ready-mix concrete or pre-mixed bags (80lb or 60lb) you need for your project. Accurate estimation prevents expensive waste or the nightmare of running short during a pour.

Please enter valid positive numbers for all fields.

How to Calculate Concrete for a Slab

Calculating the amount of concrete required for a slab involves determining the volume in cubic feet and then converting it to cubic yards, which is the standard unit for ordering ready-mix concrete trucks.

The formula is:

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

Since thickness is usually measured in inches, you must divide the inches by 12 to get feet. For example, a 4-inch slab is 0.33 feet thick.

Cubic Yards vs. Pre-Mix Bags

Cubic Yards: Used for large projects (driveways, foundations). One cubic yard is equal to 27 cubic feet. If your project requires more than 1 or 2 yards, ordering a truck is usually more economical.

Pre-Mix Bags: Ideal for small projects (walkways, post holes).

• An 80lb bag typically yields approximately 0.60 cubic feet of cured concrete.
• A 60lb bag typically yields approximately 0.45 cubic feet.

Why Include a Waste Factor?

Professional contractors always include a safety margin, typically 5% to 10%. This accounts for:

• Spillage during transport or pouring.
• Uneven subgrade (ground) that may require more depth in spots.
• Settling of the formwork.

Running out of concrete during a pour creates a "cold joint," which weakens the slab and looks unsightly. It is always better to have slightly too much than not enough.

Standard Thickness Guide

• 4 Inches: Standard for residential sidewalks, patios, and garage floors (passenger cars).
• 5-6 Inches: Recommended for heavier vehicle driveways or hot tub pads.
• 6+ Inches: Heavy equipment pads or commercial use.

function calculateConcrete() { // Get input values var length = parseFloat(document.getElementById('slabLength').value); var width = parseFloat(document.getElementById('slabWidth').value); var thicknessInches = parseFloat(document.getElementById('slabThickness').value); var wastePercent = parseFloat(document.getElementById('wasteFactor').value); var resultDiv = document.getElementById('result-display'); var errorDiv = document.getElementById('error-message'); // Validation if (isNaN(length) || isNaN(width) || isNaN(thicknessInches) || isNaN(wastePercent) || length <= 0 || width <= 0 || thicknessInches <= 0) { errorDiv.style.display = 'block'; resultDiv.style.display = 'none'; return; } errorDiv.style.display = 'none'; // Calculations // 1. Convert thickness to feet var thicknessFeet = thicknessInches / 12; // 2. Calculate Cubic Feet var cubicFeetRaw = length * width * thicknessFeet; // 3. Add Waste var wasteMultiplier = 1 + (wastePercent / 100); var totalCubicFeet = cubicFeetRaw * wasteMultiplier; // 4. Convert to Cubic Yards (1 Yard = 27 Cubic Feet) var totalCubicYards = totalCubicFeet / 27; // 5. Calculate Bags // Standard Yield: 80lb bag ~= 0.6 cu ft, 60lb bag ~= 0.45 cu ft var bags80 = totalCubicFeet / 0.6; var bags60 = totalCubicFeet / 0.45; // Update DOM document.getElementById('res-cubic-yards').innerText = totalCubicYards.toFixed(2); document.getElementById('res-cubic-feet').innerText = totalCubicFeet.toFixed(2); document.getElementById('res-80lb').innerText = Math.ceil(bags80); // Always round up bags document.getElementById('res-60lb').innerText = Math.ceil(bags60); // Always round up bags // Show results resultDiv.style.display = 'block'; }