How Much Concrete for a 20x24 Slab?
A 20×24 ft slab covers 480 sq ft. At the standard 4-inch thickness it needs 5.93 cubic yards of concrete – about 294 bags of 80 lb mix including 10% extra. Adjust the thickness below for your project.
Concrete for a 20x24 slab by thickness
| Thickness | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags | Ready-mix cost* |
|---|---|---|---|---|---|
| 4 in | 5.93 | 6.52 | 294 | 392 | $1,076 |
| 5 in | 7.41 | 8.15 | 367 | 489 | $1,344 |
| 6 in | 8.89 | 9.78 | 440 | 587 | $1,613 |
*At about $165 per cubic yard before delivery and short-load fees. Bag counts include 10% extra.
How it is calculated
20 ft × 24 ft × (4 in ÷ 12) = 160 cu ft
160 cu ft ÷ 27 = 5.93 cubic yards
80 lb bags = 160 × 1.10 ÷ 0.60 = 294
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 20x24 slab?
A 20x24 slab (480 sq ft) needs 5.93 cubic yards at 4 inches thick, 7.41 yd³ at 5 inches and 8.89 yd³ at 6 inches, before waste.
How many bags of concrete for a 20x24 slab?
At 4 inches thick with 10% extra, you need about 294 bags of 80 lb concrete mix or 392 bags of 60 lb mix.
Should I use bags or ready-mix for a 20x24 slab?
At 6.5 cubic yards, ready-mix delivery is usually faster and more consistent than mixing 294 bags by hand. Many suppliers charge a short-load fee under 3–4 yards.