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How Much Concrete for a 20×20 Slab?
4.94cubic yards
at 4 inches thick — order 5.43 yd³ with waste, or 223 × 80 lb bags
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| Thickness | Concrete needed | Order (with 10% waste) | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 in | 4.94 yd³ | 5.43 yd³ | 223 | 297 |
| 5 in | 6.17 yd³ | 6.79 yd³ | 278 | 371 |
| 6 in | 7.41 yd³ | 8.15 yd³ | 334 | 445 |
The math: 20 ft × 20 ft × thickness in feet ÷ 27 = cubic yards. A 20×20 slab is 400 square feet.
At 4 inches this pour is 223 80 lb bags — at that quantity, ready-mix delivery is almost always cheaper and far easier than bagged concrete. Patios and walkways are fine at 4 inches; use 5–6 inches where vehicles will drive.
Other slab sizes
8×8 slab10×10 slab10×12 slab12×12 slab10×20 slab12×16 slab16×16 slab12×24 slab24×24 slab20×30 slab30×30 slab24×36 slab
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