A 12 × 12 slab at 4, 5 and 6 inches
Exact volume, the order rounded up to the next ¼ yd³ with 10% extra, and bags including the extra.
| Thickness | ft³ | Exact yd³ | Order yd³ | 80-lb bags | 60-lb bags | 50-lb bags | 40-lb bags |
|---|---|---|---|---|---|---|---|
| 4 in | 48 | 1.78 | 2 | 88 | 118 | 141 | 176 |
| 5 in | 60 | 2.22 | 2.5 | 110 | 147 | 176 | 220 |
| 6 in | 72 | 2.67 | 3 | 132 | 176 | 212 | 264 |
A 12 × 12 slab in metric
12 × 12 ft is about 3.66 m × 3.66 m, or 13.38 m². Here is the same slab at the common metric thicknesses, with 10% extra and the order rounded up to the next 0.1 m³.
| Thickness | Exact m³ | Order m³ | 30-kg bags | 25-kg bags | 20-kg bags |
|---|---|---|---|---|---|
| 100 mm | 1.338 | 1.5 | 104 | 127 | 164 |
| 125 mm | 1.672 | 1.9 | 130 | 159 | 205 |
| 150 mm | 2.007 | 2.3 | 156 | 191 | 246 |
Forms and a thickened edge
The edges of a 12 × 12 slab add up to 2 × (12 + 12) = 48 ft, so that's how many linear feet of form boards you need to go all the way around. The boards must be at least as tall as the slab is thick.
Many slabs are poured with a deeper edge around the outside. If yours has one, for example an edge 12 in deep and 12 in wide under a 4 in slab, the 12 × 12 needs 2.86 cubic yards instead of 1.78: 1.09 yd³ more. With 10% extra, order 3.25 yd³ or buy 142 80-lb bags. Use the slab calculator and choose "Slab with thickened edge" to enter your own edge size.
How a 12 × 12 compares
At 4 in thick, one cubic yard of concrete covers 81 ft². A 12 × 12 slab is 144 ft², so it needs 1.78 cubic yards, more than one full yard.
- A 10 × 12 slab needs 1.48 yd³ (74 80-lb bags), which is 0.3 yd³ less than a 12 × 12.
- A 12 × 16 slab needs 2.37 yd³ (118 80-lb bags), which is 0.59 yd³ more than a 12 × 12.
Going from 4 in to 6 in thick adds 0.89 yd³ to a 12 × 12, or 44 more 80-lb bags, so settle the thickness before you order.
Bags or ready-mix for a 12 × 12?
At this size, ready-mix is usually more practical than mixing bags. Ask suppliers about truck sizes and delivery fees.
At 4 in, the 88 80-lb bags weigh 7,040 lb dry. At 6 in it's 132 bags. Ready-mix trucks carry 8 to 12 cubic yards (NRMCA), so a 12 × 12 needs one truck at 4 in. Enter your local prices in the calculator to compare costs.
Building codes set 3½ in as the minimum for slab-on-ground floors; the calculator warns you if you go thinner.
Other slab sizes
Questions people ask
How many yards of concrete for a 12 × 12 slab?
A 12 × 12 ft slab is 1.78 cubic yards at 4 in thick, 2.22 at 5 in and 2.67 at 6 in. With 10% extra, order 2, 2.5 or 3 cubic yards.
How many bags of concrete for a 12 × 12 slab?
At 4 in thick with 10% extra: 88 80-lb bags, 118 60-lb bags or 176 40-lb bags. At 6 in: 132 80-lb bags.
How much does a 12 × 12 slab weigh?
About 7,192 lb (3.6 tons) at 4 in thick, using 2,400 kg/m³ (about 150 lb/ft³).
How much concrete for a 12 × 12 slab in cubic meters?
12 × 12 ft is about 3.66 m × 3.66 m (13.38 m²). At 100 mm thick that's 1.338 m³; with 10% extra, order 1.5 m³ or buy 127 25-kg bags.
How many feet of form boards for a 12 × 12 slab?
The perimeter is 2 × (12 + 12) = 48 ft, so you need 48 linear feet of form boards around the edge, at least as tall as the slab is thick.
How many square feet is a 12 × 12 slab?
144 ft². At 4 in thick that's 48 ft³ of concrete, which is 1.78 cubic yards (cubic feet ÷ 27).
Related calculators
Sources
- QUIKRETE Concrete Mix No. 1101 product data sheet, retrieved October 2, 2026
- NRMCA, Concrete in Practice 31: Ordering Ready Mixed Concrete, retrieved October 2, 2026
- International Residential Code 2021, Section R506.1, retrieved October 2, 2026
- NRMCA, Concrete in Practice 36: Structural Lightweight Concrete, retrieved October 2, 2026
Last updated October 2, 2026. Read how we calculate, including formulas, waste defaults and rounding.