Concrete for a 12×20 slab
2.96 cubic yards (147 × 80 lb bags)
for a 12×20 ft slab 4 inches thick: 80 cu ft. Bag counts include 10% extra; at 6 inches you need 4.44 yards.
A 12×20 foot slab covers 240 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 80 cubic feet of concrete, or 2.96 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 3.5 yards, roughly $578 before delivery fees, against $956 for 147 bags of 80 lb mix.
Concrete for a 12×20 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 2.22 | 60 | 110 | 147 | 220 |
| 4 in | 2.96 | 80 | 147 | 196 | 294 |
| 5 in | 3.7 | 100 | 184 | 245 | 367 |
| 6 in | 4.44 | 120 | 220 | 294 | 440 |
| 8 in | 5.93 | 160 | 294 | 392 | 587 |
Bag counts include 10% extra for spillage and uneven ground. Cubic yards and cubic feet are the exact volume.
How it’s worked out
- Convert the thickness to feet: 4 in ÷ 12 = 0.333 ft.
- Multiply length × width × thickness: 20 × 12 × 0.333 = 80 cu ft.
- Divide by 27 for cubic yards: 80 ÷ 27 = 2.963 cu yd.
- Add 10% and divide by the bag yield: 88 ÷ 0.6 = 147 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 2.96 cubic yards. For driveways and anything that carries a vehicle, go to 5 or 6 inches and add rebar or wire mesh.
Change the numbers
Other slab sizes
- 12×18 slab
- 12×22 slab
- 10×20 slab
- 14×20 slab
- 12×16 slab
- 12×24 slab
- 12×15 slab
- 12×25 slab
- 15×20 slab
- 12×14 slab
- 12×26 slab
- 8×20 slab
Questions
- How many bags of concrete do I need for a 12×20 slab?
- At 4 inches thick, a 12×20 slab takes 147 bags of 80 lb concrete mix, 196 bags of 60 lb mix or 294 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 12×20 slab?
- 2.96 cubic yards at 4 inches thick, 3.7 at 5 inches and 4.44 at 6 inches, before adding extra for waste.
- How much does a 12×20 concrete slab cost?
- The concrete alone for a 4 inch 12×20 slab costs about $956 in 147 bags of 80 lb mix (at about $7 a bag) or about $578 for 3.5 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $1,440 to $2,880 for 240 sq ft.
- How much does a 12×20 slab weigh?
- About 5.8 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).