How Much Concrete for an 18x35 Slab?
An 18 by 35 foot slab is 630 square feet. How much concrete for an 18x35 slab at the common 4-inch thickness: about 7.78 cubic yards. Most people order 8.75 yards to cover waste and an uneven base.
Concrete calculator →The math for an 18x35 slab
- Area: 18 ft × 35 ft = 630 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 630 × 0.333 = 210.0 cubic feet.
- Cubic yards: 210.0 ÷ 27 = 7.78 yd³.
- With 10% extra: 8.56 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 18x35 Driveway?
Concrete for an 18x35 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 210.0 | 7.78 | 8.56 | 350 | 467 |
| 5 in | 262.5 | 9.72 | 10.69 | 438 | 584 |
| 6 in | 315.0 | 11.67 | 12.83 | 525 | 700 |
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 Many Tons of Asphalt for a 18x35 Driveway?
Truck or bags?
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.
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.
Joints, mesh and rebar
Cut or tool control joints so the slab cracks where you want it to. A common rule is spacing them, in feet, at no more than two to three times the slab thickness in inches, so about 8 feet apart for a 4-inch slab, and about a quarter of the slab depth deep.
Questions people ask
How many yards of concrete for an 18x35 slab at 4 inches?
About 7.78 cubic yards, or 8.56 with 10 percent extra.
How many yards for an 18x35 slab at 6 inches?
About 11.67 cubic yards.
How many 80 lb bags for an 18x35 slab?
About 350 bags at 4 inches thick, before any waste allowance.
How many square feet is an 18x35 slab?
630 square feet.
Should I add extra for waste?
Yes. Adding 5 to 10 percent covers uneven base, spillage and form bulges.
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 18x30 Slab?
- How Much Concrete for an 18x32 Slab?
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