How Much Concrete for a 12x18 Slab?
How much concrete for a 12x18 slab depends on thickness. The slab covers 12 × 18 = 216 square feet, and at 4 inches thick that is about 2.67 cubic yards, or 2.93 yards with a 10 percent allowance. Mixed from bags, it is roughly 120 bags of 80 lb mix.
Concrete calculator →The math for a 12x18 slab
- Area: 12 ft × 18 ft = 216 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 216 × 0.333 = 72.0 cubic feet.
- Cubic yards: 72.0 ÷ 27 = 2.67 yd³.
- With 10% extra: 2.93 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 12x18 Patio?
Concrete for a 12x18 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 72.0 | 2.67 | 2.93 | 120 | 160 |
| 5 in | 90.0 | 3.33 | 3.67 | 150 | 200 |
| 6 in | 108.0 | 4.00 | 4.40 | 180 | 240 |
Bag counts assume about 0.60 cubic feet per 80 lb bag and 0.45 per 60 lb bag, rounded up.
How thick to pour it
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 Gravel for a 12x18 Patio?
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.
Base and forms
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, mesh and rebar
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 12x18 slab at 4 inches?
About 2.67 cubic yards, or 2.93 with 10 percent extra.
How many yards for a 12x18 slab at 6 inches?
About 4.00 cubic yards.
How many 80 lb bags for a 12x18 slab?
About 120 bags at 4 inches thick, before any waste allowance.
How many square feet is a 12x18 slab?
216 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 a 12x14 Slab?
- How Much Concrete for a 12x15 Slab?
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