Concrete for a 18x18 Slab
Order 4.40 yd with 10% waste — or 180 × 80 lb bags / 240 × 60 lb bags.
By thickness
| Thickness | Cubic yards | +10% waste | 80 lb bags | Typical use |
|---|---|---|---|---|
| 4" | 4.00 | 4.40 | 180 | Patio, shed pad, walkway |
| 5" | 5.00 | 5.50 | 225 | Light vehicle traffic |
| 6" | 6.00 | 6.60 | 270 | Driveway, garage |
Bags or the truck?
For this 324 sq ft slab at 4": bags ≈ $1,170 (180 × 80 lb — that's 14,400 lb of mixing) vs ready-mix ≈ $695 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 18x18 slab?
A 18x18 slab at the standard 4-inch thickness needs 4.00 cubic yards of concrete — order 4.40 yards with a 10% waste margin. At 6 inches it needs 6.00 yards.
How many bags of concrete for a 18x18 slab?
At 4 inches thick: 180 bags of 80 lb concrete mix or 240 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 18x18 slab?
Ready-mix. Bags for this slab would cost about $1,170 in material and require mixing 14,400 lb by hand; a delivered order runs roughly $695 and arrives ready to pour.