How Much Concrete for a 12x20 Slab?
A 12×20 ft slab covers 240 sq ft. At the standard 4-inch thickness it needs 2.96 cubic yards of concrete – about 147 bags of 80 lb mix including 10% extra. Adjust the thickness below for your project.
Concrete for a 12x20 slab by thickness
| Thickness | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags | Ready-mix cost* |
|---|---|---|---|---|---|
| 4 in | 2.96 | 3.26 | 147 | 196 | $538 |
| 5 in | 3.7 | 4.07 | 184 | 245 | $672 |
| 6 in | 4.44 | 4.89 | 220 | 294 | $807 |
*At about $165 per cubic yard before delivery and short-load fees. Bag counts include 10% extra.
How it is calculated
12 ft × 20 ft × (4 in ÷ 12) = 80 cu ft
80 cu ft ÷ 27 = 2.96 cubic yards
80 lb bags = 80 × 1.10 ÷ 0.60 = 147
Which thickness do you need?
- 4 inches – patios, walkways, shed floors and most residential garage floors.
- 5–6 inches – driveways, hot tub pads, and garages for heavier trucks or RVs.
- Add 4–6 inches of compacted gravel base underneath and use rebar or wire mesh for slabs larger than about 10×10 ft.
Frequently asked questions
How many yards of concrete for a 12x20 slab?
A 12x20 slab (240 sq ft) needs 2.96 cubic yards at 4 inches thick, 3.7 yd³ at 5 inches and 4.44 yd³ at 6 inches, before waste.
How many bags of concrete for a 12x20 slab?
At 4 inches thick with 10% extra, you need about 147 bags of 80 lb concrete mix or 196 bags of 60 lb mix.
Should I use bags or ready-mix for a 12x20 slab?
At 3.3 cubic yards, ready-mix delivery is usually faster and more consistent than mixing 147 bags by hand. Many suppliers charge a short-load fee under 3–4 yards.