Concrete for a 18×25 slab
5.56 cubic yards (275 × 80 lb bags)
for a 18×25 ft slab 4 inches thick: 150 cu ft. Bag counts include 10% extra; at 6 inches you need 8.33 yards.
A 18×25 foot slab covers 450 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 150 cubic feet of concrete, or 5.56 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 6.25 yards, roughly $1,031 before delivery fees, against $1,788 for 275 bags of 80 lb mix.
Concrete for a 18×25 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 4.17 | 112.5 | 207 | 275 | 413 |
| 4 in | 5.56 | 150 | 275 | 367 | 550 |
| 5 in | 6.94 | 187.5 | 344 | 459 | 688 |
| 6 in | 8.33 | 225 | 413 | 550 | 825 |
| 8 in | 11.11 | 300 | 550 | 734 | 1100 |
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 × 18 × 0.333 = 150 cu ft.
- Divide by 27 for cubic yards: 150 ÷ 27 = 5.556 cu yd.
- Add 10% and divide by the bag yield: 165 ÷ 0.6 = 275 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 5.56 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
- 18×24 slab
- 18×26 slab
- 16×25 slab
- 20×25 slab
- 18×22 slab
- 18×28 slab
- 15×25 slab
- 18×20 slab
- 18×30 slab
- 14×25 slab
- 22×25 slab
- 18×18 slab
Questions
- How many bags of concrete do I need for a 18×25 slab?
- At 4 inches thick, a 18×25 slab takes 275 bags of 80 lb concrete mix, 367 bags of 60 lb mix or 550 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 18×25 slab?
- 5.56 cubic yards at 4 inches thick, 6.94 at 5 inches and 8.33 at 6 inches, before adding extra for waste.
- How much does a 18×25 concrete slab cost?
- The concrete alone for a 4 inch 18×25 slab costs about $1,788 in 275 bags of 80 lb mix (at about $7 a bag) or about $1,031 for 6.25 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $2,700 to $5,400 for 450 sq ft.
- How much does a 18×25 slab weigh?
- About 10.9 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).