How Many Bags of Concrete Do I Need for a Slab?
Bagged concrete is convenient for small jobs such as a shed base, a patio or a path. The number of bags depends on the volume of the slab and on how much concrete each bag makes, called its yield.
Step 1: find the volume
Multiply length by width by thickness, with everything in feet. A thickness of 4 inches is 4 ÷ 12 = 0.333 ft. A 10 × 10 ft slab that is 4 inches thick has a volume of 10 × 10 × 0.333 = 33.3 cubic feet.
Step 2: divide by the bag yield
Typical yields are about 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. Check the figure printed on your brand, since it varies slightly. For the 10 × 10 ft slab:
- 80 lb bags: 33.3 ÷ 0.60 = 55.6, so 56 bags
- 60 lb bags: 33.3 ÷ 0.45 = 74.1, so 75 bags
- 40 lb bags: 33.3 ÷ 0.30 = 111.1, so 112 bags
Bags needed for common 4 inch slabs
Whole bags, rounded up, with no waste added.
| Slab size | Volume | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 10 × 10 ft | 33.3 ft³ | 1.23 yd³ | 56 | 75 | 112 |
| 12 × 12 ft | 48.0 ft³ | 1.78 yd³ | 80 | 107 | 160 |
| 12 × 16 ft | 64.0 ft³ | 2.37 yd³ | 107 | 143 | 214 |
| 16 × 20 ft | 106.7 ft³ | 3.95 yd³ | 178 | 238 | 356 |
| 20 × 20 ft | 133.3 ft³ | 4.94 yd³ | 223 | 297 | 445 |
Add a margin
Ground is rarely perfectly level and forms leak a little, so most people add about 5 to 10% on top. Running out partway through a pour is far worse than having one spare bag.
Bags or ready-mix?
One cubic yard is 27 cubic feet, which is about 45 bags of 80 lb mix. Mixing that many by hand is slow, heavy work, and a pour that stops partway can leave a weak joint. For anything beyond a small slab, ready-mix delivered by truck is usually easier, and many suppliers set a minimum order.
Do not forget the base
Slabs normally sit on compacted gravel. The gravel is a separate order, so estimate it with the Gravel & Mulch Calculator.
For the full calculation, including round footings, metric units, weight and cost, use the Concrete Calculator.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard? About 45, since an 80 lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.
How many bags do I need for a 10 × 10 slab? For a 4 inch slab, about 56 bags of 80 lb mix, 75 bags of 60 lb mix or 112 bags of 40 lb mix, before adding any extra.
How thick should a concrete slab be? Four inches is common for patios and walkways. Driveways and areas with heavy loads are usually thicker, so follow local guidance for your project.
Should I buy extra bags? Yes. Add about 5 to 10% for uneven ground and spillage.