Short answer: Multiply length × width × thickness (all in feet), divide by 27 to get cubic yards, then add 10% extra. A 10 ft × 10 ft slab that is 4 inches thick needs about 1.23 cubic yards, so order roughly 1.35 yards.
If you are asking “how much concrete do I need,” the answer depends on the shape and thickness of your project. Getting it right saves money and prevents a half-finished pour.
How Is Concrete Measured?
Concrete is sold by volume. In the US, the unit is the cubic yard. One cubic yard equals 27 cubic feet. In metric countries, the unit is the cubic meter, which equals about 35.3 cubic feet.
The Basic Concrete Formula
For slabs, patios, driveways, and walkways:
Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = cubic yards
Convert thickness from inches to feet by dividing by 12. Four inches equals 0.33 feet.
Example: A 12 ft × 12 ft patio, 4 inches thick: 12 × 12 × 0.33 = 47.5 cubic feet. Divide by 27 to get 1.76 cubic yards. Adding 10% gives about 1.95 yards.
Quick Reference Chart (4 Inch Slab)
| Slab Size | Cubic Yards Needed | With 10% Extra |
| 10 × 10 ft | 1.23 | 1.35 |
| 12 × 12 ft | 1.78 | 1.96 |
| 20 × 20 ft | 4.94 | 5.43 |
Recommended Thickness by Project
- Walkways and patios: 4 inches
- Garage floors: 4 to 6 inches
- Driveways: 5 to 6 inches
- Shed foundations: 4 inches
How to Calculate Round Columns
Use this formula: π × radius² × height ÷ 27. A column that is 12 inches wide and 8 feet tall needs about 0.23 cubic yards.
Why Add 10% Extra?
The ground is rarely perfectly level. Forms can bulge, and some concrete is lost to spills. A 10% buffer prevents a short pour, which can create weak cold joints where new concrete meets old.
Bags or Truck?
An 80 lb bag of concrete mix yields about 0.6 cubic feet. One cubic yard needs roughly 45 bags. For jobs under half a yard, bags are practical. For anything larger, ready mix concrete delivery is faster, more consistent, and often cheaper once you count labor and mixing time.
Tips for Ordering Ready Mix Concrete
- Confirm your measurements twice before calling.
- Ask about minimum load fees. Small orders often carry a short-load charge.
- Make sure the truck can reach your site. A full truck can weigh over 60,000 lb.
- Have forms, rebar, and helpers ready before the truck arrives. You typically have about 60 minutes of working time.
Conclusion
To answer “how much concrete do I need,” measure carefully, apply the formula, and add 10%. Then compare bags against ready mix concrete delivery based on your total volume.