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.
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