How many bags of concrete for a 20×20 slab?
Quick Answer
A 20×20 foot slab at 4 inches thick requires approximately 6.17 cubic yards of concrete, which equals about 278 bags of 80-lb mix. At this volume, ready-mix concrete is strongly recommended — it costs about $150–180 per cubic yard delivered vs. $7 per bag ($1,946 total for bags).
Quick Reference Table
| Thickness | Concrete for 20×20 slab |
|---|---|
| 3.5 inches (minimal) | 5.4 yd³ / 243 bags 80-lb |
| 4 inches (standard) | 6.17 yd³ / 278 bags 80-lb |
| 5 inches (heavy use) | 7.7 yd³ / 347 bags 80-lb |
| 6 inches (garage) | 9.26 yd³ / 417 bags 80-lb |
How to Calculate It Yourself
- 1
Calculate volume: 20 × 20 × (4 ÷ 12) = 400 × 0.333 = 133.3 cubic feet.
- 2
Convert to cubic yards: 133.3 ÷ 27 = 4.94 yd³. Add 10% waste: × 1.10 = 5.43 yd³.
- 3
In bags: 5.43 yd³ × 45 bags/yd³ = 244 bags of 80-lb.
- 4
Call a ready-mix supplier: 5.5–6 yards delivered costs $825–1,080 vs. $1,700+ in bags.
Pro Tip
For a 20×20 slab, always use ready-mix. Mixing 250+ bags by hand takes 15+ hours and produces inconsistent concrete quality. A 6-yard ready-mix truck can pour it in under 2 hours.
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