A 12x12 slab poured 4 inches thick needs 1.78 cubic yards of concrete (48 cubic feet: 12 ft × 12 ft × 4 in ÷ 12). With 10 percent extra you order 2 cubic yards of ready-mix, or buy 88 bags of 80 lb mix and mix it yourself.
A 12x12 slab suits a patio with a table for four, a larger shed or a workshop floor, and the square shape keeps the joint layout simple.
How much concrete for a 12x12 slab, by thickness
Volume is the area times the thickness in feet. The 12x12 slab covers 144 sq ft. Ordered volumes include 10 percent extra for uneven sub-base and spillage, the middle of the 4 to 10 percent NRMCA range, and ready-mix is sold in quarter yards.
| Build | Concrete, net | Order with 10% extra | Trucks | Weight |
|---|---|---|---|---|
| 4 in, plain | 1.78 cu yd (48 cu ft) | 2 cu yd | 1 (short load) | 7,392 lb |
| 4 in, wire mesh | 1.78 cu yd (48 cu ft) | 2 cu yd | 1 (short load) | 7,392 lb |
| 6 in, rebar | 2.67 cu yd (72 cu ft) | 3 cu yd | 1 (short load) | 11,088 lb |
The weight uses 140 lb per cubic foot, Quikrete's figure for the mix. A 6 inch slab is half again the concrete of a 4 inch one for the same footprint, which is worth spending on where a car or a hot tub will sit and not on a walkway.
How many bags of concrete for a 12x12 slab
Bag counts come from the yields on the manufacturers' data sheets: an 80 lb bag makes 0.60 cu ft, 60 lb 0.45, 50 lb 0.375 and 40 lb 0.30. They include the 10 percent allowance.
| Bag size | 4 in slab | 6 in slab |
|---|---|---|
| 80 lb | 88 bags | 132 bags |
| 60 lb | 118 bags | 176 bags |
| 50 lb | 141 bags | 212 bags |
| 40 lb | 176 bags | 264 bags |
88 bags of 80 lb is about 3.5 tons of bagged mix. It is a big day with a mixer and two helpers, but it avoids a delivery minimum. Compare it against ready-mix, which for this order is a short load. The concrete bag calculator converts any bag size and brand.
12x12 slab cost, three builds
Installed prices at US averages: ready-mix $110 to $150 per cu yd, delivery, fuel and a short-load fee below a full truck, placing and finishing $2 to $3 per sq ft (HomeAdvisor, June 2026), wire mesh $0.83 to $0.98 per sq ft (homewyse, September 2026) and rebar $2 to $3 per sq ft (This Old House, March 2026).
| Build | Materials only | Installed range | Typical per sq ft | Crew days |
|---|---|---|---|---|
| 4 in, plain | $380 to $530 | $660 to $960 (typical $810) | $6 | 1 |
| 4 in, wire mesh | $490 to $670 | $780 to $1,100 (typical $940) | $7 | 1 |
| 6 in, rebar | $810 to $1,150 | $1,100 to $1,600 (typical $1,350) | $9 | 1 |
A 2 cu yd order is well below the 8 to 12 yards a truck carries, so the estimate includes a short-load fee of about $100 ($80 to $120). Splitting a truck with a neighbor who is also pouring avoids it. The heavy build adds 18 bars of 20 ft No. 4 rebar in an 18 in grid, about 240 lb with laps and 10 percent spare; the rebar calculator lays out the grid.
Joints, thickness and permit
The 2024 IRC sets 3½ in as the minimum for a slab on ground (R506.1) over 4 in of granular base with a vapor retarder under floors. NRMCA advises control joints no farther apart than 24 times the thickness: 8 ft for a 4 in slab, 12 ft for a 6 in slab. On a 12x12 slab that means 1 joint each way at 4 in thick. Under IRC 2024 R105.2, sidewalks and driveways need no permit, and a free-standing patio slab is usually treated the same way. A slab that carries a building, such as a 12x12 garage or shed floor, is part of that building's permit.
Forming, placing and finishing is about 1 day for a two-person crew. Concrete reaches 2,500 psi at 7 days and 4,000 psi at 28 days (Quikrete data sheet), so keep vehicles off for the first week.
12x12 slab compared with other sizes
A 10x10 slab saves 0.54 cu yd less, $200 less installed. A 20x20 slab needs 3.16 cu yd more, $1,250 more installed.
Next steps for a 12x12 slab
- Change thickness, shape or add a thickened edge in the concrete slab calculator.
- Work out the rebar for the slab at your own spacing.
- Turn the yards into a truck order with the concrete yard calculator, and add pumping or finishes in the concrete cost calculator.
- Any other shape or a footing is in the concrete calculator. All size pages are grouped under cost calculators.