Concrete for a 20×25 slab
6.17 cubic yards (306 × 80 lb bags)
for a 20×25 ft slab 4 inches thick: 166.7 cu ft. Bag counts include 10% extra; at 6 inches you need 9.26 yards.
A 20×25 foot slab covers 500 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 166.7 cubic feet of concrete, or 6.17 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 7 yards, roughly $1,155 before delivery fees, against $1,989 for 306 bags of 80 lb mix.
Concrete for a 20×25 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 4.63 | 125 | 230 | 306 | 459 |
| 4 in | 6.17 | 166.7 | 306 | 408 | 612 |
| 5 in | 7.72 | 208.3 | 382 | 510 | 764 |
| 6 in | 9.26 | 250 | 459 | 612 | 917 |
| 8 in | 12.35 | 333.3 | 612 | 815 | 1223 |
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: 25 × 20 × 0.333 = 166.67 cu ft.
- Divide by 27 for cubic yards: 166.67 ÷ 27 = 6.173 cu yd.
- Add 10% and divide by the bag yield: 183.33 ÷ 0.6 = 306 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 6.17 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
- 20×24 slab
- 20×26 slab
- 18×25 slab
- 22×25 slab
- 20×22 slab
- 20×28 slab
- 16×25 slab
- 20×20 slab
- 20×30 slab
- 24×25 slab
- 15×25 slab
- 25×25 slab
Questions
- How many bags of concrete do I need for a 20×25 slab?
- At 4 inches thick, a 20×25 slab takes 306 bags of 80 lb concrete mix, 408 bags of 60 lb mix or 612 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 20×25 slab?
- 6.17 cubic yards at 4 inches thick, 7.72 at 5 inches and 9.26 at 6 inches, before adding extra for waste.
- How much does a 20×25 concrete slab cost?
- The concrete alone for a 4 inch 20×25 slab costs about $1,989 in 306 bags of 80 lb mix (at about $7 a bag) or about $1,155 for 7 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $3,000 to $6,000 for 500 sq ft.
- How much does a 20×25 slab weigh?
- About 12.1 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).