How Much Concrete for an 8x40 Slab?
How much concrete for an 8x40 slab depends on thickness. The slab covers 8 × 40 = 320 square feet, and at 4 inches thick that is about 3.95 cubic yards, or 4.35 yards with a 10 percent allowance. Mixed from bags, it is roughly 178 bags of 80 lb mix.
Concrete calculator →The math for an 8x40 slab
- Area: 8 ft × 40 ft = 320 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 320 × 0.333 = 106.7 cubic feet.
- Cubic yards: 106.7 ÷ 27 = 3.95 yd³.
- With 10% extra: 4.35 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 12x20 Slab?
Concrete for an 8x40 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 106.7 | 3.95 | 4.35 | 178 | 238 |
| 5 in | 133.3 | 4.94 | 5.43 | 223 | 297 |
| 6 in | 160.0 | 5.93 | 6.52 | 267 | 356 |
Bag counts assume about 0.60 cubic feet per 80 lb bag and 0.45 per 60 lb bag, rounded up.
Choosing the thickness
Four inches is the usual thickness for patios, walks and shed floors. Slabs that carry cars or a heavy hot tub are often poured at 5 to 6 inches, and edges are sometimes thickened where loads concentrate.
Related: How Much Concrete for a 12x24 Slab?
Placing the order
Ready-mix is priced and batched by the cubic yard (27 cubic feet). Plants commonly add a small-load fee when an order is under a set minimum, so ask about it when you call. Ordering a little extra costs far less than a second truck.
Prep before the pour
Prep matters as much as the yardage. Strip sod and soft soil, compact the subgrade, then add a compacted stone base, commonly about 4 inches. A level, firm base also keeps your thickness even, which keeps the yardage close to the math.
Joints and reinforcement
Plan the joints before the pour. Panels close to square crack less than long, narrow ones, and joint spacing around 8 feet is typical for a 4-inch slab. Cut joints with a groover while the concrete is soft or with a saw within the first day.
Questions people ask
How many yards of concrete for an 8x40 slab at 4 inches?
About 3.95 cubic yards, or 4.35 with 10 percent extra.
How many yards for an 8x40 slab at 6 inches?
About 5.93 cubic yards.
How many 80 lb bags for an 8x40 slab?
About 178 bags at 4 inches thick, before any waste allowance.
How many square feet is an 8x40 slab?
320 square feet.
How many 80 lb bags make a cubic yard?
About 45. Each 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How thick should the base under the slab be?
About 4 inches of compacted gravel is common for residential slabs, more on soft or poorly drained soil.
Related guides
- How Much Area Does a Yard of Concrete Cover?
- How Many Bags of Concrete in a Yard? 40, 60, 80 lb
- Gravel Base Under a Concrete Slab: Depth and Tons
- How Many Yards in a Concrete Truck? Mixer Sizes
- Concrete Short Load Fee: What It Is and How to Cut It
- 60 lb vs 80 lb Concrete Bags: Yield, Count, and Cost
- Rebar vs Wire Mesh in a Slab: Which One to Use
- Cost to Pour a Concrete Slab: Per Square Foot
- How Much Concrete for an 8x35 Slab?
- How Much Concrete for an 8x36 Slab?
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