Concrete for a 20×32 slab
7.9 cubic yards (392 × 80 lb bags)
for a 20×32 ft slab 4 inches thick: 213.3 cu ft. Bag counts include 10% extra; at 6 inches you need 11.85 yards.
A 20×32 foot slab covers 640 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 213.3 cubic feet of concrete, or 7.9 cubic yards. At this size ready-mix is cheaper and far less work than bags: about 8.75 yards, roughly $1,444 before delivery fees, against $2,548 for 392 bags of 80 lb mix.
Concrete for a 20×32 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 5.93 | 160 | 294 | 392 | 587 |
| 4 in | 7.9 | 213.3 | 392 | 522 | 783 |
| 5 in | 9.88 | 266.7 | 489 | 652 | 978 |
| 6 in | 11.85 | 320 | 587 | 783 | 1174 |
| 8 in | 15.8 | 426.7 | 783 | 1043 | 1565 |
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: 32 × 20 × 0.333 = 213.33 cu ft.
- Divide by 27 for cubic yards: 213.33 ÷ 27 = 7.901 cu yd.
- Add 10% and divide by the bag yield: 234.67 ÷ 0.6 = 392 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 7.9 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
- 20×30 slab
- 18×32 slab
- 22×32 slab
- 20×28 slab
- 20×36 slab
- 20×26 slab
- 16×32 slab
- 24×32 slab
- 20×25 slab
- 15×32 slab
- 20×24 slab
- 20×40 slab
Questions
- How many bags of concrete do I need for a 20×32 slab?
- At 4 inches thick, a 20×32 slab takes 392 bags of 80 lb concrete mix, 522 bags of 60 lb mix or 783 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 20×32 slab?
- 7.9 cubic yards at 4 inches thick, 9.88 at 5 inches and 11.85 at 6 inches, before adding extra for waste.
- How much does a 20×32 concrete slab cost?
- The concrete alone for a 4 inch 20×32 slab costs about $2,548 in 392 bags of 80 lb mix (at about $7 a bag) or about $1,444 for 8.75 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $3,840 to $7,680 for 640 sq ft.
- How much does a 20×32 slab weigh?
- About 15.5 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).