Concrete for a 30×40 slab
14.81 cubic yards (734 × 80 lb bags)
for a 30×40 ft slab 4 inches thick: 400 cu ft. Bag counts include 10% extra; at 6 inches you need 22.22 yards.
A 30×40 foot slab covers 1200 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 400 cubic feet of concrete, or 14.81 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 16.5 yards, roughly $2,723 before delivery fees, against $4,771 for 734 bags of 80 lb mix.
Concrete for a 30×40 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 11.11 | 300 | 550 | 734 | 1100 |
| 4 in | 14.81 | 400 | 734 | 978 | 1467 |
| 5 in | 18.52 | 500 | 917 | 1223 | 1834 |
| 6 in | 22.22 | 600 | 1100 | 1467 | 2200 |
| 8 in | 29.63 | 800 | 1467 | 1956 | 2934 |
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 × 30 × 0.333 = 400 cu ft.
- Divide by 27 for cubic yards: 400 ÷ 27 = 14.815 cu yd.
- Add 10% and divide by the bag yield: 440 ÷ 0.6 = 734 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 14.81 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
- 28×40 slab
- 32×40 slab
- 30×36 slab
- 26×40 slab
- 25×40 slab
- 24×40 slab
- 30×32 slab
- 36×40 slab
- 30×30 slab
- 30×50 slab
- 22×40 slab
- 28×30 slab
Questions
- How many bags of concrete do I need for a 30×40 slab?
- At 4 inches thick, a 30×40 slab takes 734 bags of 80 lb concrete mix, 978 bags of 60 lb mix or 1467 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 30×40 slab?
- 14.81 cubic yards at 4 inches thick, 18.52 at 5 inches and 22.22 at 6 inches, before adding extra for waste.
- How much does a 30×40 concrete slab cost?
- The concrete alone for a 4 inch 30×40 slab costs about $4,771 in 734 bags of 80 lb mix (at about $7 a bag) or about $2,723 for 16.5 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $7,200 to $14,400 for 1200 sq ft.
- How much does a 30×40 slab weigh?
- About 29 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).