Concrete for a 18×28 slab
6.22 cubic yards (308 × 80 lb bags)
for a 18×28 ft slab 4 inches thick: 168 cu ft. Bag counts include 10% extra; at 6 inches you need 9.33 yards.
A 18×28 foot slab covers 504 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 168 cubic feet of concrete, or 6.22 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 7 yards, roughly $1,155 before delivery fees, against $2,002 for 308 bags of 80 lb mix.
Concrete for a 18×28 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 4.67 | 126 | 231 | 308 | 462 |
| 4 in | 6.22 | 168 | 308 | 411 | 616 |
| 5 in | 7.78 | 210 | 385 | 514 | 770 |
| 6 in | 9.33 | 252 | 462 | 616 | 924 |
| 8 in | 12.44 | 336 | 616 | 822 | 1232 |
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: 28 × 18 × 0.333 = 168 cu ft.
- Divide by 27 for cubic yards: 168 ÷ 27 = 6.222 cu yd.
- Add 10% and divide by the bag yield: 184.8 ÷ 0.6 = 308 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 6.22 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
- 18×26 slab
- 18×30 slab
- 18×25 slab
- 16×28 slab
- 20×28 slab
- 18×24 slab
- 18×32 slab
- 15×28 slab
- 18×22 slab
- 14×28 slab
- 22×28 slab
- 18×20 slab
Questions
- How many bags of concrete do I need for a 18×28 slab?
- At 4 inches thick, a 18×28 slab takes 308 bags of 80 lb concrete mix, 411 bags of 60 lb mix or 616 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 18×28 slab?
- 6.22 cubic yards at 4 inches thick, 7.78 at 5 inches and 9.33 at 6 inches, before adding extra for waste.
- How much does a 18×28 concrete slab cost?
- The concrete alone for a 4 inch 18×28 slab costs about $2,002 in 308 bags of 80 lb mix (at about $7 a bag) or about $1,155 for 7 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $3,024 to $6,048 for 504 sq ft.
- How much does a 18×28 slab weigh?
- About 12.2 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).