Concrete for a 15×18 slab
3.33 cubic yards (165 × 80 lb bags)
for a 15×18 ft slab 4 inches thick: 90 cu ft. Bag counts include 10% extra; at 6 inches you need 5 yards.
A 15×18 foot slab covers 270 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 90 cubic feet of concrete, or 3.33 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 3.75 yards, roughly $619 before delivery fees, against $1,073 for 165 bags of 80 lb mix.
Concrete for a 15×18 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 2.5 | 67.5 | 124 | 165 | 248 |
| 4 in | 3.33 | 90 | 165 | 220 | 330 |
| 5 in | 4.17 | 112.5 | 207 | 275 | 413 |
| 6 in | 5 | 135 | 248 | 330 | 495 |
| 8 in | 6.67 | 180 | 330 | 440 | 660 |
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 × 15 × 0.333 = 90 cu ft.
- Divide by 27 for cubic yards: 90 ÷ 27 = 3.333 cu yd.
- Add 10% and divide by the bag yield: 99 ÷ 0.6 = 165 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 3.33 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
- 14×18 slab
- 16×18 slab
- 15×16 slab
- 15×20 slab
- 15×15 slab
- 12×18 slab
- 18×18 slab
- 14×15 slab
- 15×22 slab
- 10×18 slab
- 12×15 slab
- 15×24 slab
Questions
- How many bags of concrete do I need for a 15×18 slab?
- At 4 inches thick, a 15×18 slab takes 165 bags of 80 lb concrete mix, 220 bags of 60 lb mix or 330 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 15×18 slab?
- 3.33 cubic yards at 4 inches thick, 4.17 at 5 inches and 5 at 6 inches, before adding extra for waste.
- How much does a 15×18 concrete slab cost?
- The concrete alone for a 4 inch 15×18 slab costs about $1,073 in 165 bags of 80 lb mix (at about $7 a bag) or about $619 for 3.75 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $1,620 to $3,240 for 270 sq ft.
- How much does a 15×18 slab weigh?
- About 6.5 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).