Concrete for a 3×20 slab
0.74 cubic yards (37 × 80 lb bags)
for a 3×20 ft slab 4 inches thick: 20 cu ft. Bag counts include 10% extra; at 6 inches you need 1.11 yards.
A 3×20 foot slab covers 60 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 20 cubic feet of concrete, or 0.74 cubic yards. That is a small enough pour for bagged mix: 37 bags of 80 lb at about $241.
Concrete for a 3×20 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 0.56 | 15 | 28 | 37 | 55 |
| 4 in | 0.74 | 20 | 37 | 49 | 74 |
| 5 in | 0.93 | 25 | 46 | 62 | 92 |
| 6 in | 1.11 | 30 | 55 | 74 | 110 |
| 8 in | 1.48 | 40 | 74 | 98 | 147 |
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: 20 × 3 × 0.333 = 20 cu ft.
- Divide by 27 for cubic yards: 20 ÷ 27 = 0.741 cu yd.
- Add 10% and divide by the bag yield: 22 ÷ 0.6 = 37 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 0.74 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
- 3×18 slab
- 3×22 slab
- 3×16 slab
- 3×24 slab
- 3×15 slab
- 3×25 slab
- 3×14 slab
- 3×26 slab
- 4×20 slab
- 3×12 slab
- 3×28 slab
- 3×10 slab
Questions
- How many bags of concrete do I need for a 3×20 slab?
- At 4 inches thick, a 3×20 slab takes 37 bags of 80 lb concrete mix, 49 bags of 60 lb mix or 74 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 3×20 slab?
- 0.74 cubic yards at 4 inches thick, 0.93 at 5 inches and 1.11 at 6 inches, before adding extra for waste.
- How much does a 3×20 concrete slab cost?
- The concrete alone for a 4 inch 3×20 slab costs about $241 in 37 bags of 80 lb mix (at about $7 a bag) or about $165 for 1 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $360 to $720 for 60 sq ft.
- How much does a 3×20 slab weigh?
- About 1.5 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).