How Much Concrete for a 12×12 Slab?
Quick Answer
For a 12×12 foot slab at standard 4-inch thickness, you need 1.78 cubic yards of concrete — approximately 80 bags of 80-lb mix or 107 bags of 60-lb mix.
- 4 inches thick: 1.78 cubic yards = 80 bags (80 lb) or 107 bags (60 lb)
- 5 inches thick: 2.22 cubic yards = 100 bags (80 lb) or 134 bags (60 lb)
- 6 inches thick: 2.67 cubic yards = 120 bags (80 lb) or 160 bags (60 lb)
- For 2+ cubic yards, consider ready-mix delivery — it is more cost-effective
Formula
Multiply 12 ft × 12 ft × thickness in inches, divide by 12 to convert to feet, then divide by 27 for cubic yards.
Step-by-Step Calculation
- 1
Convert thickness to feet
4 inches ÷ 12 = 0.333 feet
- 2
Calculate cubic feet
12 × 12 × 0.333 = 48 cubic feet
- 3
Convert to cubic yards
48 ÷ 27 = 1.78 cubic yards
- 4
Calculate 80-lb bags
48 ÷ 0.6 = 80 bags
12×12 Slab — Concrete by Thickness
| Thickness | Cubic Yards | 80-lb Bags | 60-lb Bags | Est. Cost* |
|---|---|---|---|---|
| 3 inches | 1.33 | 60 | 80 | $260–$300 |
| 4 inches | 1.78 | 80 | 107 | $345–$400 |
| 5 inches | 2.22 | 100 | 134 | $435–$500 |
| 6 inches | 2.67 | 120 | 160 | $520–$600 |
*Cost based on $4.50–$5.00 per 80-lb bag. Ready-mix delivery at $120–$160/CY may be cheaper for 2+ yards.
Frequently Asked Questions
Should I use bags or ready-mix for a 12×12 slab?
At 1.78 cubic yards (4-inch thick), you are at the threshold. 80 bags is a lot of mixing. Ready-mix delivery ($120–$160/CY) is usually more convenient and produces a stronger, more consistent pour.
How thick should a 12×12 garage slab be?
Garage slabs should be 6 inches thick minimum with #4 rebar on 18-inch centers. This supports vehicle weight and provides adequate strength for long-term use.
How much does a 12×12 concrete slab cost installed?
Professionally installed, a 12×12 slab (144 sq ft) costs $800–$1,500 for a 4-inch thick slab or $1,200–$2,100 for 6-inch thick, including materials, labor, and finishing.