Concrete for a 15×15 slab
2.78 cubic yards (138 × 80 lb bags)
for a 15×15 ft slab 4 inches thick: 75 cu ft. Bag counts include 10% extra; at 6 inches you need 4.17 yards.
A 15×15 foot slab covers 225 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 75 cubic feet of concrete, or 2.78 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 3.25 yards, roughly $536 before delivery fees, against $897 for 138 bags of 80 lb mix.
Concrete for a 15×15 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 2.08 | 56.3 | 104 | 138 | 207 |
| 4 in | 2.78 | 75 | 138 | 184 | 275 |
| 5 in | 3.47 | 93.8 | 172 | 230 | 344 |
| 6 in | 4.17 | 112.5 | 207 | 275 | 413 |
| 8 in | 5.56 | 150 | 275 | 367 | 550 |
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: 15 × 15 × 0.333 = 75 cu ft.
- Divide by 27 for cubic yards: 75 ÷ 27 = 2.778 cu yd.
- Add 10% and divide by the bag yield: 82.5 ÷ 0.6 = 138 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 2.78 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
- 14×15 slab
- 15×16 slab
- 12×15 slab
- 15×18 slab
- 10×15 slab
- 15×20 slab
- 8×15 slab
- 15×22 slab
- 6×15 slab
- 15×24 slab
- 5×15 slab
- 15×25 slab
Questions
- How many bags of concrete do I need for a 15×15 slab?
- At 4 inches thick, a 15×15 slab takes 138 bags of 80 lb concrete mix, 184 bags of 60 lb mix or 275 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 15×15 slab?
- 2.78 cubic yards at 4 inches thick, 3.47 at 5 inches and 4.17 at 6 inches, before adding extra for waste.
- How much does a 15×15 concrete slab cost?
- The concrete alone for a 4 inch 15×15 slab costs about $897 in 138 bags of 80 lb mix (at about $7 a bag) or about $536 for 3.25 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $1,350 to $2,700 for 225 sq ft.
- How much does a 15×15 slab weigh?
- About 5.4 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).