How Much Concrete for an 8x20 Slab?
An 8 by 20 foot slab is 160 square feet. How much concrete for an 8x20 slab at the common 4-inch thickness: about 1.98 cubic yards. Most people order 2.25 yards to cover waste and an uneven base.
Concrete calculator →The math for an 8x20 slab
- Area: 8 ft × 20 ft = 160 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 160 × 0.333 = 53.3 cubic feet.
- Cubic yards: 53.3 ÷ 27 = 1.98 yd³.
- With 10% extra: 2.17 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 8x20 Patio?
Concrete for an 8x20 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 53.3 | 1.98 | 2.17 | 89 | 119 |
| 5 in | 66.7 | 2.47 | 2.72 | 112 | 149 |
| 6 in | 80.0 | 2.96 | 3.26 | 134 | 178 |
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 8x20 Patio?
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.
Getting the ground ready
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.
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 10 feet apart control where cracks form.
Questions people ask
How many yards of concrete for an 8x20 slab at 4 inches?
About 1.98 cubic yards, or 2.17 with 10 percent extra.
How many yards for an 8x20 slab at 6 inches?
About 2.96 cubic yards.
How many 80 lb bags for an 8x20 slab?
About 89 bags at 4 inches thick, before any waste allowance.
How many square feet is an 8x20 slab?
160 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 an 8x16 Slab?
- How Much Concrete for an 8x18 Slab?
As an Amazon Associate, AsphaltTons.com earns from qualifying purchases. Product links below are sponsored.