How Much Concrete for a 20x20 Slab? (4 Inch Thick)

A 20 × 20 ft slab poured 4 inches thick needs 5.43 cubic yards of concrete, including 10% extra for spillage and uneven ground. That's 245 bags of 80 lb premix or 326 bags of 60 lb.

Change any number below to match your project. The concrete calculator works for any size.

What are you pouring?
For a footing, use its width, e.g. 1' 4"
Patios and walks: 4 in. Driveways: 5–6 in.
5–10% is typical

Enter your measurements to see the result.

How it's worked out

  1. Area: 20 × 20 = 400 sq ft
  2. Volume: 400 sq ft × 4 in ÷ 12 = 133.3 cu ft
  3. Add 10%: 133.3 × 1.1 = 146.7 cu ft
  4. Cubic yards: 146.7 ÷ 27 = 5.43 yd³
  5. 80 lb bags: 146.7 ÷ 0.6 cu ft per bag = 245 bags

20x20 slab at other thicknesses

ThicknessCubic yards80 lb bags60 lb bags
4 in5.43245326
5 in6.79306408
6 in8.15367489

4 inches suits patios, sheds and walkways. Driveways and garage floors usually get 5 to 6 inches.

At 5.43 yards, ready-mix delivery is usually cheaper and much less work than mixing 245 bags by hand. Most suppliers deliver in quarter or half yard steps, so round up when you order.

Other common sizes

More detail: How Many Bags of Concrete Do I Need? (Chart + Examples)