How to use it
- Enter the slab length and width.
- Enter the slab thickness — usually 4 in.
- If the edges are thickened, enter the extra depth. The calculator assumes a 12 in wide band around the perimeter.
- Keep 5–10% for waste and uneven grade.
The formula
Main slab = length × width × thickness
Thickened edge = (perimeter − 4 × band width) × band width × extra depth
Total cubic yards = (slab + edge) × (1 + waste) ÷ 27
Worked example
| Slab length | 20 ft |
| Slab width | 12 ft |
| Slab thickness | 4 in |
| Thickened edge (extra depth at perimeter) | 0 in |
| Waste / spillage allowance | 10 % |
Result
| Concrete needed | 3.26 cubic yards |
| Cubic feet | 88 ft³ |
| Cubic meters | 2.49 m³ |
| 80 lb bags | 147 bags |
| 60 lb bags | 196 bags |
| Approx. weight | 6.6 tons |
| Slab area | 240 ft² |
Tips before you order
- Compact the subgrade and lay 4 in of gravel base under the slab to prevent cracking.
- Place control joints at intervals of 2–3 times the slab thickness in feet (8–12 ft for a 4 in slab).
- Check that the subgrade is level: every ½ in of extra depth over 100 ft² adds about 0.15 yd³.
See typical concrete prices for every state on the concrete prices by state page.
Questions
How much concrete do I need for a 10×10 slab?
A 10 × 10 ft slab at 4 in thick needs 1.23 yd³ (33.3 ft³). With 10% waste, order about 1.4 yd³ or 62 bags of 80 lb concrete.
How much concrete for a 24×24 garage slab?
At 4 in thick, 24 × 24 ft needs 7.1 yd³; at 6 in it needs 10.7 yd³ before waste.