How much concrete for a 20×20 slab?
Quick Answer
A 20×20 foot slab at 4 inches thick requires 4.94 cubic yards of concrete. At 6 inches thick: 7.41 cubic yards. Always use ready-mix concrete for a 20×20 slab — you need at least 5 yards, which equals 225+ bags of 80-lb mix. Ready-mix will cost $750–1,000 vs $1,500+ in bags.
Quick Reference Table
| 20×20 slab thickness | Concrete needed |
|---|---|
| 4 inches | 4.94 yd³ |
| 5 inches | 6.17 yd³ |
| 6 inches | 7.41 yd³ |
| 8 inches | 9.88 yd³ |
| 4" + 10% waste | 5.43 yd³ → order 5.5 yd³ |
| 6" + 10% waste | 8.15 yd³ → order 8.5 yd³ |
How to Calculate It Yourself
- 1
Volume: 20 × 20 × (4/12) = 133 cubic feet.
- 2
Convert to yards: 133 ÷ 27 = 4.94 yd³.
- 3
Add 10% for waste and overpour: 4.94 × 1.10 = 5.43 yd³.
- 4
Order 5.5 yards from a ready-mix plant. Schedule pour for early morning — concrete is harder to work as it heats up.
Pro Tip
For a 20×20 slab, hire at least 3 people for the pour: one managing the chute, one screeding, one floating the edges. A 5-yard pour gives you roughly 45–60 minutes of working time in mild weather. Have everything staged and ready before the truck arrives.
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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