How to Calculate Concrete for a Slab
Multiply the slab's length × width × thickness (all in feet) and divide by 27 to convert to cubic yards. Always add 10% extra for waste, spillage, and slight variations in the subgrade. For example, a 20 × 10 foot slab at 4 inches thick requires: 20 × 10 × 0.333 / 27 = 2.47 cubic yards, plus 10% = 2.72 cubic yards. If using bags, one 80-lb bag yields approximately 0.6 cubic feet of mixed concrete.
Concrete Thickness Guide
Use 4 inches for patios, walkways, and shed floors. Use 6 inches for driveways and garage floors. Use 8 inches or more for areas supporting heavy equipment or vehicles. Thicker slabs require rebar or wire mesh reinforcement to prevent cracking.