How Many Bags of Concrete for a 3x4 Slab?
How many bags of concrete for a 3x4 slab: about 7 bags of 80 lb mix at 4 inches thick, or 9 bags of 60 lb mix. The slab is 12 square feet, which at 4 inches is 4.00 cubic feet of concrete.
Concrete calculator →How the bag count is figured
- Area: 3 × 4 = 12 sq ft.
- Volume at 4 in: 12 × 0.333 = 4.00 cu ft.
- 80 lb bags: 4.00 ÷ 0.60 = 6.7, round up to 7.
- 60 lb bags: 4.00 ÷ 0.45 = 8.9, round up to 9.
Check the yield printed on your brand’s bag, since it varies a little. The concrete calculator handles other shapes.
Related: How Many Bags of Concrete for a 10x10 Slab?
Bags by thickness for a 3x4 slab
| Thickness | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 3 in | 3.00 | 5 | 7 | 10 |
| 4 in | 4.00 | 7 | 9 | 14 |
| 5 in | 5.00 | 9 | 12 | 17 |
| 6 in | 6.00 | 10 | 14 | 20 |
When bags make sense
At 0.15 cubic yards, this pad is well under the point where a ready-mix truck starts to make sense. Many plants charge a small-load fee on tiny orders, so bags are often cheaper here, and a rented mixer makes 7 bags manageable for two people.
Related: How Many Bags of Concrete for an 8x10 Slab?
Prep for a small slab
A small slab still needs a firm base. Compact the soil, add compacted gravel and set forms to the finished height. If the pad will hold an AC unit, a generator or a grill, keep the top a little above the surrounding grade so water does not pool against it. Keep the fresh slab damp or covered for several days so it cures instead of drying out.
Buying the bags
A pallet of 80 lb bags often holds around 40 to 42 bags, so this job is about 1 pallet or less. Buy one or two extra bags: returning an unopened bag is easier than running out mid-pour.
Questions people ask
How many 80 lb bags for a 3x4 slab?
About 7 bags at 4 inches thick.
How many 60 lb bags for a 3x4 slab?
About 9 bags at 4 inches thick.
How many bags for a 3x4 slab 6 inches thick?
About 10 bags of 80 lb mix or 14 of 60 lb.
How much does an 80 lb bag of concrete yield?
About 0.6 cubic feet of mixed concrete.
How thick should the base under the slab be?
About 4 inches of compacted gravel is common for residential slabs, more on soft or poorly drained soil.
Related guides
- How Much Area Does a Yard of Concrete Cover?
- How Many Bags of Concrete in a Yard? 40, 60, 80 lb
- Gravel Base Under a Concrete Slab: Depth and Tons
- How Many Yards in a Concrete Truck? Mixer Sizes
- Concrete Short Load Fee: What It Is and How to Cut It
- 60 lb vs 80 lb Concrete Bags: Yield, Count, and Cost
- Rebar vs Wire Mesh in a Slab: Which One to Use
- How Many Bags of Concrete for a 2x3 Slab?
- How Many Bags of Concrete for a 3x3 Slab?
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