How much concrete do I need for a 12×12 slab?
A 12 × 12 ft slab poured 4 inches thick needs 1.96 cubic yards of concrete, which is 89 bags of 80 lb mix or 118 bags of 60 lb. That includes 10 percent extra for an uneven base; the exact volume is 1.78 cubic yards (48.0 cubic feet). At 6 inches, for a driveway or garage, it is 2.93 cubic yards or 132 bags of 80 lb.
Concrete for a 12×12 slab by thickness
| Thickness | Exact volume | Order (10% extra) | 80 lb bags | 60 lb bags | 50 lb bags | 40 lb bags |
|---|---|---|---|---|---|---|
| 4 in | 1.78 cu yd (48.0 cu ft) | 1.96 cu yd | 89 | 118 | 141 | 177 |
| 5 in | 2.22 cu yd (60.0 cu ft) | 2.44 cu yd | 110 | 147 | 176 | 220 |
| 6 in | 2.67 cu yd (72.0 cu ft) | 2.93 cu yd | 132 | 176 | 212 | 264 |
Bag counts are rounded up and include the same 10 percent allowance. One 80 lb bag yields 0.60 cubic feet; 60 lb yields 0.45; 50 lb yields 0.375; 40 lb yields 0.30.
Change the numbers
The calculator below is preset for this size. Adjust anything and the results update instantly.
Bags or a ready-mix truck for a 12×12 slab?
This slab sits in the gray zone. Bags are possible (89 of 80 lb, about $578 and 4.5 hours of mixing), but a ready-mix truck delivers 1.96 yards for roughly $244 to $342 plus a short-load fee of $75 to $150, and the whole pour is finished in an hour. Most people go with the truck here; a slab this size poured in batches from bags risks cold joints between batches.
Everything else a 12×12 slab needs
- Gravel base: 4 in of compacted crushed stone over 144 sq ft is 1.96 cubic yards, about 2.9 tons. See the gravel calculator.
- Reinforcement: wire mesh in 5 × 10 ft sheets (4 sheets) or #4 rebar on a 16 in grid, about 207 linear feet. The rebar calculator prices it out.
- Forms: 48 linear feet of 2 × 4 (7 eight-footers) with a stake every 2 ft, about 24 stakes.
- Vapor barrier: 166 sq ft of 6 mil plastic if the slab is for a shed, garage, or anything enclosed.
- Control joints: cut or tool 2 joints so no panel is longer than 10 ft. Cut within 12 hours of finishing.
- Expansion joint: 1/2 in fiber strip wherever the slab meets a house, driveway, or other concrete.
How the numbers are figured
12 × 12 ft is 144 square feet. Multiply by the thickness in feet (4 in is 0.333 ft) for cubic feet, divide by 27 for cubic yards, and add 10 percent because a gravel base is never perfectly flat. Bag counts divide the cubic feet by each bag's yield. The concrete calculator above does the same math for any size, shape, or waste factor.
Common questions
How many 80 lb bags of concrete for a 12x12 slab?
89 bags at 4 inches thick, 110 bags at 5 inches, or 132 bags at 6 inches, each including 10 percent extra. In 60 lb bags the 4 inch slab takes 118.
How much does concrete for a 12x12 slab cost?
About $578 in 80 lb bags at $6.50 each, or $244 to $342 for 1.96 yards of ready-mix before any short-load fee. Having a contractor form, pour, and finish it typically runs $864 to $1,440, or $6 to $10 per square foot.
How thick should a 12x12 concrete slab be?
4 inches for a patio, walkway, shed floor, or hot tub pad on well-compacted gravel. Go to 5 or 6 inches if cars will drive on it, and 6 inches with rebar for a garage, workshop with heavy tools, or RV pad. For a 12×12 slab the jump from 4 to 6 inches adds 0.98 cubic yards.