How Much Concrete for a 16x20 Slab?
A 16×20 ft slab covers 320 sq ft. At the standard 4-inch thickness it needs 3.95 cubic yards of concrete – about 196 bags of 80 lb mix including 10% extra. Adjust the thickness below for your project.
Concrete for a 16x20 slab by thickness
| Thickness | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags | Ready-mix cost* |
|---|---|---|---|---|---|
| 4 in | 3.95 | 4.35 | 196 | 261 | $717 |
| 5 in | 4.94 | 5.43 | 245 | 326 | $896 |
| 6 in | 5.93 | 6.52 | 294 | 392 | $1,076 |
*At about $165 per cubic yard before delivery and short-load fees. Bag counts include 10% extra.
How it is calculated
16 ft × 20 ft × (4 in ÷ 12) = 106.7 cu ft
106.7 cu ft ÷ 27 = 3.95 cubic yards
80 lb bags = 106.7 × 1.10 ÷ 0.60 = 196
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 16x20 slab?
A 16x20 slab (320 sq ft) needs 3.95 cubic yards at 4 inches thick, 4.94 yd³ at 5 inches and 5.93 yd³ at 6 inches, before waste.
How many bags of concrete for a 16x20 slab?
At 4 inches thick with 10% extra, you need about 196 bags of 80 lb concrete mix or 261 bags of 60 lb mix.
Should I use bags or ready-mix for a 16x20 slab?
At 4.3 cubic yards, ready-mix delivery is usually faster and more consistent than mixing 196 bags by hand. Many suppliers charge a short-load fee under 3–4 yards.