Bag coverage is controlled by the yield printed on the product, not by the bag weight alone. Measure the project volume first, then divide by the stated yield and round up.
Use the label yield
If an 80 lb bag yields 0.60 cubic feet, 10 cubic feet requires 10 ÷ 0.60 = 16.67 bags before waste, so the purchase count is at least 17. Different mixes and bag sizes have different yields.
Worked slab example
A 10 ft × 10 ft slab at 4 inches has 33.33 cubic feet. With 10% waste, the planning volume is 36.67 cubic feet. At 0.60 cubic feet per bag, 36.67 ÷ 0.60 = 61.11, so round up to 62 bags.
Why coverage changes
Uneven subgrade, spillage, forms, aggregate size, mixing loss, and depth variation can increase use. A product label may assume a particular water ratio and finish. Follow the manufacturer’s instructions.
Bag work versus ready-mix
Many bags can be labor-intensive and may require continuous mixing. Compare the material cost, labor, access, minimum ready-mix order, and placement time before choosing the method.
Sources
The figures and rules in this guide are checked against:
- QUIKRETE Concrete Mix (product data and yield) — QUIKRETE
- Concrete Calculators — QUIKRETE
- High-Strength Concrete Mix — Sakrete
- How Concrete Is Made — Portland Cement Association