Concrete for a 6×18 slab
1.33 cubic yards (66 × 80 lb bags)
for a 6×18 ft slab 4 inches thick: 36 cu ft. Bag counts include 10% extra; at 6 inches you need 2 yards.
A 6×18 foot slab covers 108 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 36 cubic feet of concrete, or 1.33 cubic yards. At this size you can mix 66 bags of 80 lb (about $429 and a long day of mixing) or order 1.5 yards of ready-mix (about $248, plus the short-load fee most suppliers charge under 3 to 4 yards).
Concrete for a 6×18 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 1 | 27 | 50 | 66 | 99 |
| 4 in | 1.33 | 36 | 66 | 88 | 132 |
| 5 in | 1.67 | 45 | 83 | 110 | 165 |
| 6 in | 2 | 54 | 99 | 132 | 198 |
| 8 in | 2.67 | 72 | 132 | 176 | 264 |
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: 18 × 6 × 0.333 = 36 cu ft.
- Divide by 27 for cubic yards: 36 ÷ 27 = 1.333 cu yd.
- Add 10% and divide by the bag yield: 39.6 ÷ 0.6 = 66 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 1.33 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
- 6×16 slab
- 6×20 slab
- 5×18 slab
- 6×15 slab
- 6×14 slab
- 6×22 slab
- 4×18 slab
- 6×12 slab
- 6×24 slab
- 8×18 slab
- 6×25 slab
- 6×10 slab
Questions
- How many bags of concrete do I need for a 6×18 slab?
- At 4 inches thick, a 6×18 slab takes 66 bags of 80 lb concrete mix, 88 bags of 60 lb mix or 132 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 6×18 slab?
- 1.33 cubic yards at 4 inches thick, 1.67 at 5 inches and 2 at 6 inches, before adding extra for waste.
- How much does a 6×18 concrete slab cost?
- The concrete alone for a 4 inch 6×18 slab costs about $429 in 66 bags of 80 lb mix (at about $7 a bag) or about $248 for 1.5 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $648 to $1,296 for 108 sq ft.
- How much does a 6×18 slab weigh?
- About 2.6 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).