Concrete for a 18×36 slab
8 cubic yards (396 × 80 lb bags)
for a 18×36 ft slab 4 inches thick: 216 cu ft. Bag counts include 10% extra; at 6 inches you need 12 yards.
A 18×36 foot slab covers 648 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 216 cubic feet of concrete, or 8 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 9 yards, roughly $1,485 before delivery fees, against $2,574 for 396 bags of 80 lb mix.
Concrete for a 18×36 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 6 | 162 | 297 | 396 | 594 |
| 4 in | 8 | 216 | 396 | 528 | 792 |
| 5 in | 10 | 270 | 495 | 660 | 990 |
| 6 in | 12 | 324 | 594 | 792 | 1188 |
| 8 in | 16 | 432 | 792 | 1056 | 1584 |
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: 36 × 18 × 0.333 = 216 cu ft.
- Divide by 27 for cubic yards: 216 ÷ 27 = 8 cu yd.
- Add 10% and divide by the bag yield: 237.6 ÷ 0.6 = 396 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 8 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
- 16×36 slab
- 18×32 slab
- 18×40 slab
- 20×36 slab
- 15×36 slab
- 18×30 slab
- 14×36 slab
- 18×28 slab
- 22×36 slab
- 18×26 slab
- 18×25 slab
- 12×36 slab
Questions
- How many bags of concrete do I need for a 18×36 slab?
- At 4 inches thick, a 18×36 slab takes 396 bags of 80 lb concrete mix, 528 bags of 60 lb mix or 792 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 18×36 slab?
- 8 cubic yards at 4 inches thick, 10 at 5 inches and 12 at 6 inches, before adding extra for waste.
- How much does a 18×36 concrete slab cost?
- The concrete alone for a 4 inch 18×36 slab costs about $2,574 in 396 bags of 80 lb mix (at about $7 a bag) or about $1,485 for 9 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $3,888 to $7,776 for 648 sq ft.
- How much does a 18×36 slab weigh?
- About 15.7 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).