How Much Concrete for an 18 Foot Round Slab?
How much concrete for an 18 foot round slab: about 3.14 cubic yards at 4 inches thick. A circle 18 feet across has a radius of 9 feet and an area of about 254.5 square feet (π × 9²).
Concrete calculator →The circle math
- Radius: 18 ÷ 2 = 9 ft.
- Area: 3.1416 × 9 × 9 = 254.5 sq ft.
- Volume at 4 in: 254.5 × 0.333 = 84.8 cu ft.
- Yards: ÷ 27 = 3.14 yd³.
A round slab takes about 21 percent less concrete than a square of the same width (18x18 would need 4.00 yards at 4 inches).
Related: How Much Concrete for a 24 Foot Round Slab?
By thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 84.8 | 3.14 | 3.46 | 142 | 189 |
| 5 in | 106.0 | 3.93 | 4.32 | 177 | 236 |
| 6 in | 127.2 | 4.71 | 5.18 | 213 | 283 |
Forming a circle
Mark the circle with a stake and a string the length of the radius. Flexible form material such as hardboard strips or plastic forms bends to the curve; stake it closely, since curves push outward when the concrete goes in.
Related: How Much Concrete for a 24 Foot Round Slab?
Common uses and thickness
Fire pit patios, gazebo floors and tank pads are the usual round slabs. For people and furniture, 4 inches on compacted gravel is standard. Anything heavy needs more thickness and possibly reinforcement; check the manufacturer’s requirements for tanks or bins.
Joints in a round slab
Radial joints from the center keep cracks in check. On larger circles, a ring joint partway out keeps the outer panels from getting too long and narrow.
Questions people ask
How many yards of concrete for an 18 ft round slab?
About 3.14 cubic yards at 4 inches.
How many square feet is an 18 ft circle?
About 254.5 square feet.
How many 80 lb bags for an 18 ft round slab?
About 142 bags at 4 inches thick.
How do I find the area of a circle?
Multiply 3.1416 by the radius squared. The radius is half the diameter.
How much does a cubic yard of concrete weigh?
Roughly 4,000 pounds, or about 2 tons, for normal-weight concrete.
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
- Gravel for a Fire Pit Area: Best Types and Depth
- How Much Concrete for a 15 Foot Round Slab?
- How Much Concrete for a 16 Foot Round Slab?
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