Concrete for a 20×50 slab
12.35 cubic yards (612 × 80 lb bags)
for a 20×50 ft slab 4 inches thick: 333.3 cu ft. Bag counts include 10% extra; at 6 inches you need 18.52 yards.
A 20×50 foot slab covers 1000 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 333.3 cubic feet of concrete, or 12.35 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 13.75 yards, roughly $2,269 before delivery fees, against $3,978 for 612 bags of 80 lb mix.
Concrete for a 20×50 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 9.26 | 250 | 459 | 612 | 917 |
| 4 in | 12.35 | 333.3 | 612 | 815 | 1223 |
| 5 in | 15.43 | 416.7 | 764 | 1019 | 1528 |
| 6 in | 18.52 | 500 | 917 | 1223 | 1834 |
| 8 in | 24.69 | 666.7 | 1223 | 1630 | 2445 |
Bag counts include 10% extra for spillage and uneven ground. Cubic yards and cubic feet are the exact volume.
How it’s worked out
- Convert the thickness to feet: 4 in ÷ 12 = 0.333 ft.
- Multiply length × width × thickness: 50 × 20 × 0.333 = 333.33 cu ft.
- Divide by 27 for cubic yards: 333.33 ÷ 27 = 12.346 cu yd.
- Add 10% and divide by the bag yield: 366.67 ÷ 0.6 = 612 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 12.35 cubic yards. For driveways and anything that carries a vehicle, go to 5 or 6 inches and add rebar or wire mesh.
Change the numbers
Other slab sizes
- 18×50 slab
- 22×50 slab
- 16×50 slab
- 20×40 slab
- 24×50 slab
- 15×50 slab
- 25×50 slab
- 20×36 slab
- 14×50 slab
- 26×50 slab
- 20×32 slab
- 12×50 slab
Questions
- How many bags of concrete do I need for a 20×50 slab?
- At 4 inches thick, a 20×50 slab takes 612 bags of 80 lb concrete mix, 815 bags of 60 lb mix or 1223 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 20×50 slab?
- 12.35 cubic yards at 4 inches thick, 15.43 at 5 inches and 18.52 at 6 inches, before adding extra for waste.
- How much does a 20×50 concrete slab cost?
- The concrete alone for a 4 inch 20×50 slab costs about $3,978 in 612 bags of 80 lb mix (at about $7 a bag) or about $2,269 for 13.75 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $6,000 to $12,000 for 1000 sq ft.
- How much does a 20×50 slab weigh?
- About 24.2 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).