How Much Concrete for a 30x40 Slab?
A 30×40 ft slab covers 1200 sq ft. At the standard 4-inch thickness it needs 14.81 cubic yards of concrete – about 734 bags of 80 lb mix including 10% extra. Adjust the thickness below for your project.
Concrete for a 30x40 slab by thickness
| Thickness | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags | Ready-mix cost* |
|---|---|---|---|---|---|
| 4 in | 14.81 | 16.3 | 734 | 978 | $2,689 |
| 5 in | 18.52 | 20.37 | 917 | 1223 | $3,361 |
| 6 in | 22.22 | 24.44 | 1100 | 1467 | $4,033 |
*At about $165 per cubic yard before delivery and short-load fees. Bag counts include 10% extra.
How it is calculated
30 ft × 40 ft × (4 in ÷ 12) = 400 cu ft
400 cu ft ÷ 27 = 14.81 cubic yards
80 lb bags = 400 × 1.10 ÷ 0.60 = 734
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 30x40 slab?
A 30x40 slab (1200 sq ft) needs 14.81 cubic yards at 4 inches thick, 18.52 yd³ at 5 inches and 22.22 yd³ at 6 inches, before waste.
How many bags of concrete for a 30x40 slab?
At 4 inches thick with 10% extra, you need about 734 bags of 80 lb concrete mix or 978 bags of 60 lb mix.
Should I use bags or ready-mix for a 30x40 slab?
At 16.3 cubic yards, ready-mix delivery is usually faster and more consistent than mixing 734 bags by hand. Many suppliers charge a short-load fee under 3–4 yards.