How Much Concrete for a 12x36 Slab?
How much concrete for a 12x36 slab depends on thickness. The slab covers 12 × 36 = 432 square feet, and at 4 inches thick that is about 5.33 cubic yards, or 5.87 yards with a 10 percent allowance. Mixed from bags, it is roughly 240 bags of 80 lb mix.
Concrete calculator →The math for a 12x36 slab
- Area: 12 ft × 36 ft = 432 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 432 × 0.333 = 144.0 cubic feet.
- Cubic yards: 144.0 ÷ 27 = 5.33 yd³.
- With 10% extra: 5.87 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 12x36 Parking Pad?
Concrete for a 12x36 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 144.0 | 5.33 | 5.87 | 240 | 320 |
| 5 in | 180.0 | 6.67 | 7.33 | 300 | 400 |
| 6 in | 216.0 | 8.00 | 8.80 | 360 | 480 |
Bag counts assume about 0.60 cubic feet per 80 lb bag and 0.45 per 60 lb bag, rounded up.
Thickness for this job
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: Recycled Concrete Aggregate for a 12x36 Parking Pad
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
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.
Keeping cracks under control
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 12x36 slab at 4 inches?
About 5.33 cubic yards, or 5.87 with 10 percent extra.
How many yards for a 12x36 slab at 6 inches?
About 8.00 cubic yards.
How many 80 lb bags for a 12x36 slab?
About 240 bags at 4 inches thick, before any waste allowance.
How many square feet is a 12x36 slab?
432 square feet.
Should I add extra for waste?
Yes. Adding 5 to 10 percent covers uneven base, spillage and form bulges.
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 12x32 Slab?
- How Much Concrete for a 12x35 Slab?
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