Concrete for a 20×40 slab
9.88 cubic yards (489 × 80 lb bags)
for a 20×40 ft slab 4 inches thick: 266.7 cu ft. Bag counts include 10% extra; at 6 inches you need 14.81 yards.
A 20×40 foot slab covers 800 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 266.7 cubic feet of concrete, or 9.88 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 11 yards, roughly $1,815 before delivery fees, against $3,179 for 489 bags of 80 lb mix.
Concrete for a 20×40 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 7.41 | 200 | 367 | 489 | 734 |
| 4 in | 9.88 | 266.7 | 489 | 652 | 978 |
| 5 in | 12.35 | 333.3 | 612 | 815 | 1223 |
| 6 in | 14.81 | 400 | 734 | 978 | 1467 |
| 8 in | 19.75 | 533.3 | 978 | 1304 | 1956 |
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 × 20 × 0.333 = 266.67 cu ft.
- Divide by 27 for cubic yards: 266.67 ÷ 27 = 9.877 cu yd.
- Add 10% and divide by the bag yield: 293.33 ÷ 0.6 = 489 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 9.88 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
- 18×40 slab
- 20×36 slab
- 22×40 slab
- 16×40 slab
- 20×32 slab
- 24×40 slab
- 15×40 slab
- 20×30 slab
- 20×50 slab
- 25×40 slab
- 14×40 slab
- 20×28 slab
Questions
- How many bags of concrete do I need for a 20×40 slab?
- At 4 inches thick, a 20×40 slab takes 489 bags of 80 lb concrete mix, 652 bags of 60 lb mix or 978 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 20×40 slab?
- 9.88 cubic yards at 4 inches thick, 12.35 at 5 inches and 14.81 at 6 inches, before adding extra for waste.
- How much does a 20×40 concrete slab cost?
- The concrete alone for a 4 inch 20×40 slab costs about $3,179 in 489 bags of 80 lb mix (at about $7 a bag) or about $1,815 for 11 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $4,800 to $9,600 for 800 sq ft.
- How much does a 20×40 slab weigh?
- About 19.3 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).