Concrete for a 12x20 Slab
Order 3.26 yd with 10% waste — or 134 × 80 lb bags / 178 × 60 lb bags.
By thickness
| Thickness | Cubic yards | +10% waste | 80 lb bags | Typical use |
|---|---|---|---|---|
| 4" | 2.96 | 3.26 | 134 | Patio, shed pad, walkway |
| 5" | 3.70 | 4.07 | 167 | Light vehicle traffic |
| 6" | 4.44 | 4.89 | 200 | Driveway, garage |
Bags or the truck?
For this 240 sq ft slab at 4": bags ≈ $871 (134 × 80 lb — that's 10,720 lb of mixing) vs ready-mix ≈ $534 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 12x20 slab?
A 12x20 slab at the standard 4-inch thickness needs 2.96 cubic yards of concrete — order 3.26 yards with a 10% waste margin. At 6 inches it needs 4.44 yards.
How many bags of concrete for a 12x20 slab?
At 4 inches thick: 134 bags of 80 lb concrete mix or 178 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 12x20 slab?
Ready-mix. Bags for this slab would cost about $871 in material and require mixing 10,720 lb by hand; a delivered order runs roughly $534 and arrives ready to pour.