How to Calculate Concrete Bags Needed for Slabs, Footings & Steps
💡 Bottom Line Up Front (BLUF): Premix Bag Requirement Table (100 sq ft)
For a standard 100 sq ft slab, the number of premix concrete bags required depends directly on slab thickness (4", 5", or 6"). Always add a 10% waste allowance for uneven sub-base excavation.
| Slab Thickness | Cubic Feet Volume | Cubic Yards | 80lb Bags (+10% Waste) | 60lb Bags (+10% Waste) |
|---|---|---|---|---|
| 4 Inches (Patio/Walkway) | 33.33 ft³ | 1.23 yd³ | 61 bags | 81 bags |
| 5 Inches (Driveway/Shed) | 41.67 ft³ | 1.54 yd³ | 76 bags | 102 bags |
| 6 Inches (Heavy Vehicle) | 50.00 ft³ | 1.85 yd³ | 92 bags | 122 bags |
1. Rectangular Slab Calculation Formula
To calculate bags for a slab, multiply length by width by thickness (in feet) and divide by the bag yield ($0.60 \text{ ft}^3$ per 80lb bag).
Bags (80lb) = (Length_ft × Width_ft × Depth_in / 12) ÷ 0.60 × 1.10
For raw material batching with cement, sand, and gravel, refer to our Concrete Mix Ratios & Compaction Guide.
2. Cylindrical Post & Sonotube Footing Formula
For round post holes and cylindrical sonotube footings, calculate volume using $V = \pi r^2 h$.
A 12-inch diameter sonotube post hole dug 3 feet deep yields $1.57 \text{ ft}^3$ of volume, requiring 3 bags of 80lb premix concrete.