Concrete for a 3×15 slab
0.56 cubic yards (28 × 80 lb bags)
for a 3×15 ft slab 4 inches thick: 15 cu ft. Bag counts include 10% extra; at 6 inches you need 0.83 yards.
A 3×15 foot slab covers 45 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 15 cubic feet of concrete, or 0.56 cubic yards. That is a small enough pour for bagged mix: 28 bags of 80 lb at about $182.
Concrete for a 3×15 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 0.42 | 11.3 | 21 | 28 | 42 |
| 4 in | 0.56 | 15 | 28 | 37 | 55 |
| 5 in | 0.69 | 18.8 | 35 | 46 | 69 |
| 6 in | 0.83 | 22.5 | 42 | 55 | 83 |
| 8 in | 1.11 | 30 | 55 | 74 | 110 |
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 × 3 × 0.333 = 15 cu ft.
- Divide by 27 for cubic yards: 15 ÷ 27 = 0.556 cu yd.
- Add 10% and divide by the bag yield: 16.5 ÷ 0.6 = 28 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 0.56 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
- 3×14 slab
- 3×16 slab
- 3×12 slab
- 3×18 slab
- 3×10 slab
- 3×20 slab
- 4×15 slab
- 3×8 slab
- 3×22 slab
- 3×6 slab
- 3×24 slab
- 3×5 slab
Questions
- How many bags of concrete do I need for a 3×15 slab?
- At 4 inches thick, a 3×15 slab takes 28 bags of 80 lb concrete mix, 37 bags of 60 lb mix or 55 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 3×15 slab?
- 0.56 cubic yards at 4 inches thick, 0.69 at 5 inches and 0.83 at 6 inches, before adding extra for waste.
- How much does a 3×15 concrete slab cost?
- The concrete alone for a 4 inch 3×15 slab costs about $182 in 28 bags of 80 lb mix (at about $7 a bag) or about $124 for 0.75 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $270 to $540 for 45 sq ft.
- How much does a 3×15 slab weigh?
- About 1.1 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).