Concrete & Masonry Guide
How Much Concrete Do I Need for a Slab?
Bag calculations, ready-mix yardage, slab thickness rules, and waste estimates for patios, sheds, and driveways.
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1. Concrete Slab Formula
To calculate cubic yards for a concrete slab:
Volume (cu ft) = Length (ft) × Width (ft) × (Thickness in inches ÷ 12)Cubic Yards = Volume (cu ft) ÷ 27Total with 10% Waste = Cubic Yards × 1.102. Common Slab Dimensions & Bag Counts (4" Slab)
| Slab Size | Cubic Yards (w/ 10% waste) | 80 lb Bags Needed | Delivery Recommendation |
|---|---|---|---|
| 8 ft × 8 ft (Shed pad) | 0.87 yd³ | 40 bags | DIY Hand Mix or Cart-Away Trailer |
| 10 ft × 10 ft (Patio) | 1.36 yd³ | 62 bags | Ready-Mix Mini Truck or Rental Mixer |
| 12 ft × 12 ft (Large Patio) | 1.96 yd³ | 88 bags | Ready-Mix Truck (2.0 yd³ batch) |
| 20 ft × 20 ft (2-Car Garage) | 5.43 yd³ | 245 bags | Ready-Mix Truck (5.5 yd³ order) |