How Many Bags of Concrete for a Slab?

Quick Answer

To find the number of 80 lb bags needed, calculate your total cubic feet and divide by 0.6. For example, a 10x10 slab at 4 inches thick is 33.3 cubic feet, which requires about 56 bags (80 lb).

Detailed Explanation

When working on smaller projects, buying pre-mixed bags of concrete (like Quikrete or Sakrete) is often more practical than ordering a delivery truck. However, mixing bags is labor-intensive, so it's crucial to know exactly how many you need.

Bag Yields

  • 80 lb bag: Yields approximately 0.60 cubic feet
  • 60 lb bag: Yields approximately 0.45 cubic feet
  • 50 lb bag: Yields approximately 0.37 cubic feet

Step-by-Step Math Breakdown (Example: 20x20 slab at 4")

1. Calculate cubic feet: 20 × 20 × (4/12) = 133.33 cubic feet
2. For 80 lb bags: 133.33 ÷ 0.60 = 222.2 bags (Round up to 223)
3. For 60 lb bags: 133.33 ÷ 0.45 = 296.3 bags (Round up to 297)

Materials Note

Mixing 223 bags of concrete by hand or with a small portable mixer is an enormous amount of work. As a general rule of thumb in the industry, if your project requires more than 30-40 bags (about 1 cubic yard), it is usually more cost-effective and significantly less labor-intensive to order ready-mix concrete delivered by a truck.

Related Tools & Guides