Round Slab Calculator
Short answer: a 12 ft diameter round slab at 4 inches thick needs 1.40 cubic yards (1.54 with 10% waste) — about 70 × 80 lb bags. Enter your diameter below.
Calculate your round slab
How the calculation works
Area = π × radius², where radius = diameter ÷ 2. Volume = area × thickness in feet (thickness in inches ÷ 12). Cubic yards = cubic feet ÷ 27.
Worked example: a 12 ft diameter slab at 4 in thick: radius 6 ft, area π × 6² = 113.10 sq ft, volume 113.10 × (4 ÷ 12) = 37.70 cu ft = 1.40 cubic yards. With 10% waste: 1.54 cubic yards, or about 70 × 80 lb bags.
Common round slabs at 4 inches thick (before waste)
| Diameter | Area | Concrete (net) | 80 lb bags (net) |
|---|---|---|---|
| 6 ft | 28.27 sq ft | 0.35 cubic yards | 16 |
| 8 ft | 50.27 sq ft | 0.62 cubic yards | 28 |
| 10 ft | 78.54 sq ft | 0.97 cubic yards | 44 |
| 12 ft | 113.10 sq ft | 1.40 cubic yards | 63 |
| 16 ft | 201.06 sq ft | 2.48 cubic yards | 112 |
| 20 ft | 314.16 sq ft | 3.88 cubic yards | 175 |
Volumes are net (before waste). Bag counts use the 0.60 cu ft yield and are rounded up.
Frequently asked questions
How much concrete do I need for a 12 foot round slab?
At 4 inches: 1.40 cubic yards net, 1.54 with 10% waste — about 70 × 80 lb bags from the order volume.
How do you calculate concrete for a round slab?
Area = π × radius², volume = area × thickness, then divide cubic feet by 27 for cubic yards. Add 5–10% waste before ordering.
Is a round slab calculated differently from a rectangular slab?
Only the area step differs — a circle uses π × radius² instead of length × width. Thickness, waste and bag math are the same.