Concrete for a 20x40 Slab
Order 10.86 yd with 10% waste — or 445 × 80 lb bags / 593 × 60 lb bags.
By thickness
| Thickness | Cubic yards | +10% waste | 80 lb bags | Typical use |
|---|---|---|---|---|
| 4" | 9.88 | 10.86 | 445 | Patio, shed pad, walkway |
| 5" | 12.35 | 13.58 | 556 | Light vehicle traffic |
| 6" | 14.81 | 16.30 | 667 | Driveway, garage |
Bags or the truck?
For this 800 sq ft slab at 4": bags ≈ $2,893 (445 × 80 lb — that's 35,600 lb of mixing) vs ready-mix ≈ $1,531 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 20x40 slab?
A 20x40 slab at the standard 4-inch thickness needs 9.88 cubic yards of concrete — order 10.86 yards with a 10% waste margin. At 6 inches it needs 14.81 yards.
How many bags of concrete for a 20x40 slab?
At 4 inches thick: 445 bags of 80 lb concrete mix or 593 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 20x40 slab?
Ready-mix. Bags for this slab would cost about $2,893 in material and require mixing 35,600 lb by hand; a delivered order runs roughly $1,531 and arrives ready to pour.