Concrete for a 8×40 slab
3.95 cubic yards (196 × 80 lb bags)
for a 8×40 ft slab 4 inches thick: 106.7 cu ft. Bag counts include 10% extra; at 6 inches you need 5.93 yards.
A 8×40 foot slab covers 320 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 106.7 cubic feet of concrete, or 3.95 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 4.5 yards, roughly $743 before delivery fees, against $1,274 for 196 bags of 80 lb mix.
Concrete for a 8×40 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 2.96 | 80 | 147 | 196 | 294 |
| 4 in | 3.95 | 106.7 | 196 | 261 | 392 |
| 5 in | 4.94 | 133.3 | 245 | 326 | 489 |
| 6 in | 5.93 | 160 | 294 | 392 | 587 |
| 8 in | 7.9 | 213.3 | 392 | 522 | 783 |
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: 40 × 8 × 0.333 = 106.67 cu ft.
- Divide by 27 for cubic yards: 106.67 ÷ 27 = 3.951 cu yd.
- Add 10% and divide by the bag yield: 117.33 ÷ 0.6 = 196 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 3.95 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
- 8×36 slab
- 8×32 slab
- 6×40 slab
- 8×30 slab
- 8×50 slab
- 10×40 slab
- 8×28 slab
- 8×26 slab
- 5×40 slab
- 8×25 slab
- 8×24 slab
- 8×22 slab
Questions
- How many bags of concrete do I need for a 8×40 slab?
- At 4 inches thick, a 8×40 slab takes 196 bags of 80 lb concrete mix, 261 bags of 60 lb mix or 392 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 8×40 slab?
- 3.95 cubic yards at 4 inches thick, 4.94 at 5 inches and 5.93 at 6 inches, before adding extra for waste.
- How much does a 8×40 concrete slab cost?
- The concrete alone for a 4 inch 8×40 slab costs about $1,274 in 196 bags of 80 lb mix (at about $7 a bag) or about $743 for 4.5 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $1,920 to $3,840 for 320 sq ft.
- How much does a 8×40 slab weigh?
- About 7.7 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).