Concrete for a 18×18 slab
4 cubic yards (198 × 80 lb bags)
for a 18×18 ft slab 4 inches thick: 108 cu ft. Bag counts include 10% extra; at 6 inches you need 6 yards.
A 18×18 foot slab covers 324 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 108 cubic feet of concrete, or 4 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 4.5 yards, roughly $743 before delivery fees, against $1,287 for 198 bags of 80 lb mix.
Concrete for a 18×18 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 3 | 81 | 149 | 198 | 297 |
| 4 in | 4 | 108 | 198 | 264 | 396 |
| 5 in | 5 | 135 | 248 | 330 | 495 |
| 6 in | 6 | 162 | 297 | 396 | 594 |
| 8 in | 8 | 216 | 396 | 528 | 792 |
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 × 18 × 0.333 = 108 cu ft.
- Divide by 27 for cubic yards: 108 ÷ 27 = 4 cu yd.
- Add 10% and divide by the bag yield: 118.8 ÷ 0.6 = 198 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 4 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
- 16×18 slab
- 18×20 slab
- 15×18 slab
- 14×18 slab
- 18×22 slab
- 12×18 slab
- 18×24 slab
- 18×25 slab
- 10×18 slab
- 18×26 slab
- 8×18 slab
- 18×28 slab
Questions
- How many bags of concrete do I need for a 18×18 slab?
- At 4 inches thick, a 18×18 slab takes 198 bags of 80 lb concrete mix, 264 bags of 60 lb mix or 396 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 18×18 slab?
- 4 cubic yards at 4 inches thick, 5 at 5 inches and 6 at 6 inches, before adding extra for waste.
- How much does a 18×18 concrete slab cost?
- The concrete alone for a 4 inch 18×18 slab costs about $1,287 in 198 bags of 80 lb mix (at about $7 a bag) or about $743 for 4.5 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $1,944 to $3,888 for 324 sq ft.
- How much does a 18×18 slab weigh?
- About 7.8 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).