How many bags of concrete for a 12×12 slab?
Quick Answer
A 12×12 foot slab poured 4 inches thick requires 1.78 cubic yards of concrete — approximately 80 bags of 80-lb mix. At 6-inch thickness you need 2.67 cubic yards, or 120 bags. For any slab this size, ready-mix concrete is significantly cheaper and faster than bags.
Quick Reference Table
| 12×12 slab thickness | 80-lb bags needed |
|---|---|
| 3 inches | 60 bags (1.33 yd³) |
| 4 inches | 80 bags (1.78 yd³) |
| 5 inches | 100 bags (2.22 yd³) |
| 6 inches | 120 bags (2.67 yd³) |
| Ready-mix (4") | 1.78 yd³ ≈ $280–350 |
| Bags cost (4") | 80 × $7 = $560+ |
How to Calculate It Yourself
- 1
Volume: 12 × 12 × (4/12) = 48 cubic feet.
- 2
Convert to yards: 48 ÷ 27 = 1.78 yd³.
- 3
Add 10% waste: 1.78 × 1.10 = 1.96 yd³. Round to 2 yards for ready-mix order.
- 4
If using bags: 1.96 ÷ 0.022 = 89 bags of 80-lb mix. Round up to 90.
Pro Tip
A 12×12 slab is 1.78 yards — right at the threshold where ready-mix makes financial and practical sense. One ready-mix truck minimum is usually 1 yard. Call a local batch plant: a 2-yard delivery ($300–400) will be cheaper than 90 bags ($630+) and take 2 hours less to pour.
Assumptions to Check
Before ordering materials, confirm the dimensions, product coverage, waste factor, and local installation requirements for your project. Manufacturer coverage tables and local code rules can change the final quantity, especially for structural work, exterior projects, and irregular layouts.
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