How to Use This Calculator
- Measure the slab perimeter: Enter length and width in feet.
- Specify slab depth: Enter thickness in inches (4 inches is standard for patios and sidewalks).
- Choose pre-mix bags or ready-mix: For jobs over 1.5 yd³, contact a ready-mix concrete batch plant.
Formula and Calculation Steps
Cubic Yards = (Length in ft × Width in ft × Thickness in inches) ÷ 324
Cubic Meters = Cubic Yards × 0.764555
60-lb Bags = Cubic Yards ÷ 0.017
80-lb Bags = Cubic Yards ÷ 0.022Worked Examples and Reference Table
| Dimensions (4" Depth) | Cubic Yards | Cubic Meters | 80-lb Bags |
|---|---|---|---|
| 4 ft × 8 ft | 0.40 yd³ | 0.31 m³ | 18 Bags |
| 10 ft × 10 ft | 1.23 yd³ | 0.94 m³ | 56 Bags |
| 12 ft × 12 ft | 1.78 yd³ | 1.36 m³ | 81 Bags |
| 20 ft × 20 ft | 4.94 yd³ | 3.78 m³ | 225 Bags (Truck) |
Frequently Asked Questions
How do you calculate concrete in cubic yards?
Multiply length (feet) by width (feet) by thickness (inches converted to feet by dividing by 12), then divide the total cubic feet by 27. The shortcut formula is: (Length in ft × Width in ft × Thickness in inches) ÷ 324.
How many bags of concrete make a cubic yard?
It takes approximately 60 bags of 60-lb pre-mix concrete, or about 45 bags of 80-lb pre-mix concrete to yield 1 cubic yard.
What is the standard thickness for a concrete slab?
Standard residential walkways and patios are typically 4 inches thick. Driveways supporting passenger vehicles should be 4 to 5 inches, while heavy truck pads require 6 inches or more.
Should I order extra concrete for a pour?
Yes. Most concrete contractors recommend adding 5% to 10% safety margin for ground spillage, excavation irregularities, and slab deflection.
When should I order ready-mix truck delivery vs pre-mix bags?
For projects requiring more than 1 to 1.5 cubic yards (around 50 to 70 bags), ordering a ready-mix concrete delivery truck is significantly more economical and saves heavy labor.