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.
Assumptions to Check
Before ordering materials, confirm the dimensions, product coverage, waste factor, and local installation requirements for your project. Manufacturer coverage tables and local code rules can change the final quantity, especially for structural work, exterior projects, and irregular layouts.
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