How Much Concrete for an 8x18 Slab?
An 8 by 18 foot slab is 144 square feet. How much concrete for an 8x18 slab at the common 4-inch thickness: about 1.78 cubic yards. Most people order 2.00 yards to cover waste and an uneven base.
Concrete calculator →The math for an 8x18 slab
- Area: 8 ft × 18 ft = 144 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 144 × 0.333 = 48.0 cubic feet.
- Cubic yards: 48.0 ÷ 27 = 1.78 yd³.
- With 10% extra: 1.96 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 8x18 Patio?
Concrete for an 8x18 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 48.0 | 1.78 | 1.96 | 80 | 107 |
| 5 in | 60.0 | 2.22 | 2.44 | 100 | 134 |
| 6 in | 72.0 | 2.67 | 2.93 | 120 | 160 |
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 Crushed Stone for a 8x18 Patio?
Placing the order
Concrete is ordered by the cubic yard, and a yard is 27 cubic feet. Ready-mix plants usually sell in quarter- or half-yard steps, so round up to the next step rather than down. Running short in the middle of a pour leaves a cold joint, which is a weak seam in the slab.
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
Reinforcement does not stop cracks, but it holds them tight. Wire mesh or rebar set mid-depth on chairs is common for a slab that carries vehicles; fiber-reinforced mix is an option for light-duty slabs. Joints about 10 feet apart control where cracks form.
Questions people ask
How many yards of concrete for an 8x18 slab at 4 inches?
About 1.78 cubic yards, or 1.96 with 10 percent extra.
How many yards for an 8x18 slab at 6 inches?
About 2.67 cubic yards.
How many 80 lb bags for an 8x18 slab?
About 80 bags at 4 inches thick, before any waste allowance.
How many square feet is an 8x18 slab?
144 square feet.
How much does a cubic yard of concrete weigh?
Roughly 4,000 pounds, or about 2 tons, for normal-weight concrete.
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 8x15 Slab?
- How Much Concrete for an 8x16 Slab?
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