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.
Pro Tips
- At 1.78 cubic yards (80 bags), you are right at the threshold where ready-mix becomes practical. If the truck can back within 20 feet of your pour site, ready-mix saves 2–3 hours of mixing labor and produces a stronger, more consistent slab.
- Score a control joint diagonally across the slab at the midpoint — a 12×12 slab needs at least one control joint to manage shrinkage cracks. Use a groover tool while the concrete is still workable, cutting to 1/4 of the slab depth.
- Rent a vibrating screed ($80–$120/day) for this size — it gives a much flatter surface than hand-screeding, especially if you are using a stiff 4-inch slump mix. The time saved in finishing alone is worth the rental.
- Order your concrete for delivery between 7–9 AM. This gives you cooler temperatures for the pour and a full working day to finish, edge, and apply curing compound before dark.
Key Cost Factors
- Ready-mix for 1.78 cubic yards at $120–$160 per yard costs $215–$285 delivered. Add $50–$100 for the short-load fee since you are under 3 yards. Compare that to 80 bags at $4.50 each ($360) — ready-mix is actually cheaper and saves hours of backbreaking mixing.
- Reinforcement adds $40–$80 to the project. Wire mesh (6×6 W1.4) costs about $40 for a 12×12 slab. Rebar (#4 at 18-inch centers) costs $60–$80 but adds significantly more structural strength, especially for garage or heavy-use slabs.
Common Mistakes to Avoid
- Assuming bags are always cheaper. At 1.78 cubic yards (80 bags), you are near the break-even point where ready-mix delivery starts winning on both cost and labor. Get a delivery quote before committing to 80 bags.
- Underestimating mixing time. Eighty 80-lb bags is 3–4 hours of continuous, physical mixing for two people. Many DIYers run out of daylight or energy mid-pour, which creates cold joints between sections.
- Ignoring the sub-base depth. A 12×12 slab covers 144 square feet, so any soft or organic soil under it will show up as differential settling. Excavate and compact a proper gravel base across the full footprint.
- Skipping reinforcement. At this size, wire mesh or #3 rebar on 18-inch centers is worth the small added cost to tie the slab together and keep shrinkage cracks tight.
Related Project Considerations
- A continuous pour beats sections. A 12×12 slab is large enough that a delayed second batch creates a cold joint — a visible line and a plane of weakness. If mixing by hand, line up enough help to pour it all within an hour.
- Control joints are essential at this size. Plan to cut joints dividing the slab into four 6×6 panels within 6–12 hours of pouring, before shrinkage cracking begins.
- Think about the end use. If this slab will hold a shed, hot tub, or vehicle, jump to 5–6 inches and add rebar rather than pouring a standard 4-inch patio slab that may crack under the load.
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 where hand-mixing gets impractical. Eighty 80-lb bags is a full day of backbreaking labor with a mixer. Ready-mix delivery at $120–$160 per cubic yard is usually more convenient, produces a stronger and more consistent pour, and lets you finish the surface before it sets — which is hard to do when you are still mixing the last batch.
How thick should a 12×12 garage slab be?
Garage slabs should be 6 inches thick minimum with #4 rebar on 18-inch centers in both directions. This supports passenger vehicles (3,000–5,000 lbs concentrated on four small tire patches) and provides adequate strength for long-term use. If you plan to park heavy trucks, install a lift, or store industrial equipment, consider going to 8 inches with a thickened edge at the perimeter for added load transfer.
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, forming, gravel base, and finishing. Costs vary by region and site conditions — rocky soil, poor access, or removal of an old slab add to the price. Get at least three quotes and verify each includes base preparation, not just the pour.