Concrete for a 20x20 Slab
Order 5.43 yd with 10% waste — or 223 × 80 lb bags / 297 × 60 lb bags.
By thickness
| Thickness | Cubic yards | +10% waste | 80 lb bags | Typical use |
|---|---|---|---|---|
| 4" | 4.94 | 5.43 | 223 | Patio, shed pad, walkway |
| 5" | 6.17 | 6.79 | 278 | Light vehicle traffic |
| 6" | 7.41 | 8.15 | 334 | Driveway, garage |
Bags or the truck?
For this 400 sq ft slab at 4": bags ≈ $1,450 (223 × 80 lb — that's 17,840 lb of mixing) vs ready-mix ≈ $840 delivered. The truck wins here. Prices are national ballparks — your local quote rules.
Adjust the numbers
Common questions
How much concrete do I need for a 20x20 slab?
A 20x20 slab at the standard 4-inch thickness needs 4.94 cubic yards of concrete — order 5.43 yards with a 10% waste margin. At 6 inches it needs 7.41 yards.
How many bags of concrete for a 20x20 slab?
At 4 inches thick: 223 bags of 80 lb concrete mix or 297 bags of 60 lb. At this volume, delivered ready-mix is usually cheaper and far less work than mixing bags.
Should I use bags or ready-mix for a 20x20 slab?
Ready-mix. Bags for this slab would cost about $1,450 in material and require mixing 17,840 lb by hand; a delivered order runs roughly $840 and arrives ready to pour.