How to Calculate Concrete Yards for a Slab: Cubic Yard Formula

Pouring a concrete patio, driveway, sidewalk, or shed foundation requires calculating the exact volume of ready-mix concrete in cubic yards. Ordering too little concrete creates cold joints that ruin structural integrity, while ordering too much wastes hundreds of dollars.

This guide explains the fundamental cubic yard conversion formula, standard slab thickness requirements, bag estimates (60 lb and 80 lb bags), and links to the CalculatorAll Concrete Slab Calculator.

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The Cubic Yards Concrete Formula

Because slab length and width are measured in feet while thickness is measured in inches, you must convert thickness to feet before calculating volume in cubic feet, then divide by 27 (since 1 cubic yard = 27 cubic feet):

Volume (Cubic Feet) = Length (ft) × Width (ft) × (Thickness in Inches / 12)

Cubic Yards = Volume (Cubic Feet) / 27

Cubic Yards with 10% Buffer = Cubic Yards × 1.10

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Step-by-Step Worked Example: 12x12 Patio Slab

Suppose you are pouring a 12 ft × 12 ft backyard patio slab with a standard 4-inch thickness:

1. Convert 4 inches to feet: 4 / 12 = 0.333 feet
2. Calculate Cubic Feet: 12 ft × 12 ft × 0.333 ft = 48.0 cubic feet
3. Convert to Cubic Yards: 48.0 / 27 = 1.78 cubic yards
4. Add 10% Waste Buffer: 1.78 × 1.10 = 1.96 cubic yards (Order 2.0 Yards)

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Bag Estimator (Premix Bags)

If you are mixing bags by hand or with a portable mixer rather than ordering a ready-mix concrete truck:

• One 80-lb bag of concrete yields: ~0.60 cubic feet (~45 bags per cubic yard).
• One 60-lb bag of concrete yields: ~0.45 cubic feet (~60 bags per cubic yard).

For the 12×12 slab (48 cubic feet):

• 80 lb Bags Needed: 48 / 0.60 = 80 bags (approx 88 bags with 10% buffer).
• 60 lb Bags Needed: 48 / 0.45 = 107 bags (approx 118 bags with 10% buffer).

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Standard Recommended Slab Thicknesses