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16×20 slab: concrete by thickness
| Thickness | Cu ft | Cu yd | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5″ | 93.33 | 3.46 | 156 | 208 |
| 4″ | 106.67 | 3.95 | 178 | 238 |
| 5″ | 133.33 | 4.94 | 223 | 297 |
| 6″ | 160.00 | 5.93 | 267 | 356 |
No waste included in the table.
Worked example
16 ft × 20 ft = 320 sq ft. At 4 inches: 320 × 4 ÷ 12 = 106.67 cu ft, or 3.95 cubic yards. With 10% extra that is 4.35 yd³, or 196 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 16×20 slab?
178 × 80 lb or 238 × 60 lb bags at 4 inches thick, before waste.
How many yards of concrete for a 16×20 slab?
3.95 cubic yards at 4 inches; 5.93 at 6 inches.