How Much Concrete for a 20x30 Slab?
A 20×30 ft slab covers 600 sq ft. At the standard 4-inch thickness it needs 7.41 cubic yards of concrete – about 367 bags of 80 lb mix including 10% extra. Adjust the thickness below for your project.
Concrete for a 20x30 slab by thickness
| Thickness | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags | Ready-mix cost* |
|---|---|---|---|---|---|
| 4 in | 7.41 | 8.15 | 367 | 489 | $1,344 |
| 5 in | 9.26 | 10.19 | 459 | 612 | $1,681 |
| 6 in | 11.11 | 12.22 | 550 | 734 | $2,017 |
*At about $165 per cubic yard before delivery and short-load fees. Bag counts include 10% extra.
How it is calculated
20 ft × 30 ft × (4 in ÷ 12) = 200 cu ft
200 cu ft ÷ 27 = 7.41 cubic yards
80 lb bags = 200 × 1.10 ÷ 0.60 = 367
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 20x30 slab?
A 20x30 slab (600 sq ft) needs 7.41 cubic yards at 4 inches thick, 9.26 yd³ at 5 inches and 11.11 yd³ at 6 inches, before waste.
How many bags of concrete for a 20x30 slab?
At 4 inches thick with 10% extra, you need about 367 bags of 80 lb concrete mix or 489 bags of 60 lb mix.
Should I use bags or ready-mix for a 20x30 slab?
At 8.1 cubic yards, ready-mix delivery is usually faster and more consistent than mixing 367 bags by hand. Many suppliers charge a short-load fee under 3–4 yards.