How to Calculate Concrete in Cubic Yards (With Common Project Sizes)
Every concrete project starts with the same formula. Here it is with ready-to-use calculations for the 10 most common projects.
The universal concrete formula: Length (feet) × Width (feet) × Thickness (feet) = Cubic Feet. Divide by 27 = Cubic Yards. Always add 10% for waste and uneven ground. For thickness in inches: divide by 12 to convert to feet first. A 10×12 slab at 4 inches: 10 × 12 × 0.333 = 40 cubic feet ÷ 27 = 1.48 cubic yards + 10% = 1.63 yards. Order 1.75 to be safe.
Common Project Sizes
Fence post hole (10 inch × 36 inch deep): 0.14 cubic feet = 2 bags of 80-lb concrete per post. Sidewalk section (3 ft × 20 ft × 4 in): 0.74 cubic yards. Patio (10 × 12 × 4 in): 1.63 yards. Single car driveway (10 × 20 × 5 in): 3.4 yards. Two-car driveway (16 × 40 × 5 in): 10.6 yards. Basketball court pad (20 × 25 × 5 in): 8.7 yards. Garage floor (20 × 24 × 4 in): 6.6 yards. Small shed pad (8 × 10 × 4 in): 1.0 yard.
Bags vs Truck
Each 80-lb bag makes 0.6 cubic feet. For 1 cubic yard you need 45 bags — that is 3,600 pounds of concrete to mix by hand. Anything over 1 yard (45 bags), order a ready-mix truck ($130-160 per yard, 1-yard minimum). The truck saves hours of back-breaking labor and produces a more consistent, higher-quality pour.