How Much Concrete for a 20x20 Slab?
A 20×20 ft slab covers 400 sq ft. At the standard 4-inch thickness it needs 4.94 cubic yards of concrete – about 245 bags of 80 lb mix including 10% extra. Adjust the thickness below for your project.
Concrete for a 20x20 slab by thickness
| Thickness | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags | Ready-mix cost* |
|---|---|---|---|---|---|
| 4 in | 4.94 | 5.43 | 245 | 326 | $896 |
| 5 in | 6.17 | 6.79 | 306 | 408 | $1,120 |
| 6 in | 7.41 | 8.15 | 367 | 489 | $1,344 |
*At about $165 per cubic yard before delivery and short-load fees. Bag counts include 10% extra.
How it is calculated
20 ft × 20 ft × (4 in ÷ 12) = 133.3 cu ft
133.3 cu ft ÷ 27 = 4.94 cubic yards
80 lb bags = 133.3 × 1.10 ÷ 0.60 = 245
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 20x20 slab?
A 20x20 slab (400 sq ft) needs 4.94 cubic yards at 4 inches thick, 6.17 yd³ at 5 inches and 7.41 yd³ at 6 inches, before waste.
How many bags of concrete for a 20x20 slab?
At 4 inches thick with 10% extra, you need about 245 bags of 80 lb concrete mix or 326 bags of 60 lb mix.
Should I use bags or ready-mix for a 20x20 slab?
At 5.4 cubic yards, ready-mix delivery is usually faster and more consistent than mixing 245 bags by hand. Many suppliers charge a short-load fee under 3–4 yards.