How Much Concrete for a 15x18 Slab?
For a 15x18 slab poured 4 inches thick, plan on about 3.33 cubic yards of concrete. That comes from 270 square feet times 0.333 feet of depth, divided by 27. How much concrete for a 15x18 slab at other thicknesses is in the table below.
Concrete calculator →The math for a 15x18 slab
- Area: 15 ft × 18 ft = 270 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 270 × 0.333 = 90.0 cubic feet.
- Cubic yards: 90.0 ÷ 27 = 3.33 yd³.
- With 10% extra: 3.67 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 12x20 Slab?
Concrete for a 15x18 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 90.0 | 3.33 | 3.67 | 150 | 200 |
| 5 in | 112.5 | 4.17 | 4.58 | 188 | 250 |
| 6 in | 135.0 | 5.00 | 5.50 | 225 | 300 |
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?
Ordering ready-mix
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
Under the slab, most residential pours sit on about 4 inches of compacted gravel. The base spreads the load, drains water away and keeps the slab from cracking as the soil freezes and thaws. Compact the base in thin lifts, and dampen it just before the pour so it does not pull water out of the mix.
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 8 feet apart control where cracks form.
Questions people ask
How many yards of concrete for a 15x18 slab at 4 inches?
About 3.33 cubic yards, or 3.67 with 10 percent extra.
How many yards for a 15x18 slab at 6 inches?
About 5.00 cubic yards.
How many 80 lb bags for a 15x18 slab?
About 150 bags at 4 inches thick, before any waste allowance.
How many square feet is a 15x18 slab?
270 square feet.
How much does a cubic yard of concrete weigh?
Roughly 4,000 pounds, or about 2 tons, for normal-weight concrete.
When is bagged concrete practical?
Usually for jobs under about a cubic yard. Beyond that, mixing dozens of bags by hand gets slow and batches may not blend well.
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 a 15x15 Slab?
- How Much Concrete for a 15x16 Slab?
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