Concrete for a 4×20 slab
0.99 cubic yards (49 × 80 lb bags)
for a 4×20 ft slab 4 inches thick: 26.7 cu ft. Bag counts include 10% extra; at 6 inches you need 1.48 yards.
A 4×20 foot slab covers 80 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 26.7 cubic feet of concrete, or 0.99 cubic yards. At this size you can mix 49 bags of 80 lb (about $319 and a long day of mixing) or order 1.25 yards of ready-mix (about $206, plus the short-load fee most suppliers charge under 3 to 4 yards).
Concrete for a 4×20 slab by thickness
| Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3 in | 0.74 | 20 | 37 | 49 | 74 |
| 4 in | 0.99 | 26.7 | 49 | 66 | 98 |
| 5 in | 1.23 | 33.3 | 62 | 82 | 123 |
| 6 in | 1.48 | 40 | 74 | 98 | 147 |
| 8 in | 1.98 | 53.3 | 98 | 131 | 196 |
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 × 4 × 0.333 = 26.67 cu ft.
- Divide by 27 for cubic yards: 26.67 ÷ 27 = 0.988 cu yd.
- Add 10% and divide by the bag yield: 29.33 ÷ 0.6 = 49 bags of 80 lb mix (rounded up).
Under the slab, plan on 4 inches of compacted gravel: about 0.99 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
- 4×18 slab
- 4×22 slab
- 4×16 slab
- 4×24 slab
- 3×20 slab
- 4×15 slab
- 4×25 slab
- 5×20 slab
- 4×14 slab
- 4×26 slab
- 4×12 slab
- 4×28 slab
Questions
- How many bags of concrete do I need for a 4×20 slab?
- At 4 inches thick, a 4×20 slab takes 49 bags of 80 lb concrete mix, 66 bags of 60 lb mix or 98 bags of 40 lb mix, including 10% extra.
- How many yards of concrete for a 4×20 slab?
- 0.99 cubic yards at 4 inches thick, 1.23 at 5 inches and 1.48 at 6 inches, before adding extra for waste.
- How much does a 4×20 concrete slab cost?
- The concrete alone for a 4 inch 4×20 slab costs about $319 in 49 bags of 80 lb mix (at about $7 a bag) or about $206 for 1.25 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $480 to $960 for 80 sq ft.
- How much does a 4×20 slab weigh?
- About 1.9 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).