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18×18 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 94.50 | 3.50 | 158 | 210 |
| 4″ | 108.00 | 4.00 | 180 | 240 |
| 5″ | 135.00 | 5.00 | 225 | 300 |
| 6″ | 162.00 | 6.00 | 270 | 360 |
No waste included in the table.
Worked example
18 ft × 18 ft = 324 sq ft. At 4 inches: 324 × 4 ÷ 12 = 108.00 cu ft, or 4.00 cubic yards. With 10% extra that is 4.40 yd³, or 198 bags of 80 lb mix.
At over 1 cubic yard, compare the cost and labor of bags against a ready-mix delivery.
Assumes standard concrete mix bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft. Check the yield on your bag. 1 cubic yard = 27 cubic feet.
Frequently asked questions
How many bags of concrete for a 18×18 slab?
180 × 80 lb or 240 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 18×18 slab?
4.00 cubic yards at 4 inches; 6.00 at 6 inches.