Concrete for a 28×40 slab
13.83 cubic yards (685 × 80 lb bags)
for a 28×40 ft slab 4 inches thick: 373.3 cu ft. Bag counts include 10% extra; at 6 inches you need 20.74 yards.
A 28×40 foot slab covers 1120 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 373.3 cubic feet of concrete, or 13.83 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 15.25 yards, roughly $2,516 before delivery fees, against $4,453 for 685 bags of 80 lb mix.
Concrete for a 28×40 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 10.37 | 280 | 514 | 685 | 1027 |
| 4 in | 13.83 | 373.3 | 685 | 913 | 1369 |
| 5 in | 17.28 | 466.7 | 856 | 1141 | 1712 |
| 6 in | 20.74 | 560 | 1027 | 1369 | 2054 |
| 8 in | 27.65 | 746.7 | 1369 | 1826 | 2738 |
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: 40 × 28 × 0.333 = 373.33 cu ft.
- Divide by 27 for cubic yards: 373.33 ÷ 27 = 13.827 cu yd.
- Add 10% and divide by the bag yield: 410.67 ÷ 0.6 = 685 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 13.83 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
- 26×40 slab
- 30×40 slab
- 28×36 slab
- 25×40 slab
- 24×40 slab
- 32×40 slab
- 28×32 slab
- 22×40 slab
- 28×30 slab
- 28×50 slab
- 20×40 slab
- 36×40 slab
Questions
- How many bags of concrete do I need for a 28×40 slab?
- At 4 inches thick, a 28×40 slab takes 685 bags of 80 lb concrete mix, 913 bags of 60 lb mix or 1369 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 28×40 slab?
- 13.83 cubic yards at 4 inches thick, 17.28 at 5 inches and 20.74 at 6 inches, before adding extra for waste.
- How much does a 28×40 concrete slab cost?
- The concrete alone for a 4 inch 28×40 slab costs about $4,453 in 685 bags of 80 lb mix (at about $7 a bag) or about $2,516 for 15.25 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $6,720 to $13,440 for 1120 sq ft.
- How much does a 28×40 slab weigh?
- About 27.1 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).