How Much Concrete for a 20x20 Slab?
A 20x20 ft slab (400 sq ft) poured 4 inches thick needs about 5.43 cubic yards of concrete, or 245 bags of 80-lb mix. Both figures include 10% extra for spillage and an uneven subgrade.
20x20 slab by thickness
| Thickness | Cubic yards | 80-lb bags | 60-lb bags | 40-lb bags | Weight |
|---|---|---|---|---|---|
| 4 in (typical patio) | 5.43 | 245 | 326 | 489 | 22,000 lb |
| 5 in | 6.79 | 306 | 408 | 612 | 27,500 lb |
| 6 in (driveway, heavy use) | 8.15 | 367 | 489 | 734 | 33,000 lb |
Volume = length × width × thickness ÷ 12 = 20 × 20 × 4 ÷ 12 = 133.3 cu ft (4.94 cu yd) before the 10% extra. Bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft.
Bags or ready-mix?
For the 4-inch slab, 245 bags at an example $7 each is about $1,715, while 5.43 yards of ready-mix at an example $165 a yard is about $896 before delivery and any short-load fee. Prices vary a lot by region and supplier, so enter your own in the concrete calculator. Mixing 245 bags means moving about 19,600 pounds of dry mix, plus water.
Before you order
A 20x20 slab also needs a base: gravel under the concrete is measured in cubic yards too, so use the gravel calculator for a 4-inch base. Check the area with the square footage calculator if the space is not a perfect rectangle, and add the extra for forms that bow or ground that dips. Wire mesh or rebar, expansion joints and curing time are not included in these figures.
Frequently asked questions
How many bags of concrete do I need for a 20x20 slab?
For a 20x20 ft slab 4 inches thick you need about 245 bags of 80-lb mix, 326 bags of 60-lb mix or 489 bags of 40-lb mix. That includes 10% extra for spillage and an uneven subgrade. Without the extra, the slab is 133.3 cubic feet.
How many yards of concrete is a 20x20 slab?
A 20x20 slab at 4 inches is 4.94 cubic yards, or 5.43 cubic yards with 10% extra. At 5 inches it is 6.79 yards and at 6 inches 8.15 yards.
How much does a 20x20 slab of concrete weigh?
Normal-weight concrete weighs about 150 pounds per cubic foot, so a 20x20 slab 4 inches thick weighs roughly 22,000 pounds (11 tons), including the extra.
Should I use bags or ready-mix for a 20x20 slab?
At 245 bags, mixing by hand is a lot of work and the slab should be poured in one go to avoid cold joints. Ordering 5.43 yards of ready-mix is usually the better choice, though small orders often carry a short-load fee.
Other slab sizes
Planning figures for a rectangular slab. Brands and delivery practices differ, so confirm quantities with your supplier.