How Many Bags or Cubic Yards of Concrete for a Slab?
October 21, 2026
The volume of a slab is length × width × thickness. A 10 ft × 10 ft slab that is 4 inches thick holds 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards. Add about 10% for uneven ground and spillage and you need roughly 1.36 cubic yards, or about 82 bags of 60 lb premix.
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Convert every dimension to feet, multiply to get cubic feet, then divide by 27 to get cubic yards. Thickness in inches must be divided by 12 first, which is the most common slip.
With a 10% waste allowance, the 33.3 cubic feet becomes 36.7 cubic feet. A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.6 cubic feet, so you need about 82 sixty-pound bags or about 62 eighty-pound bags. Always check the yield printed on the bag.
| Slab size (4 in thick) | Cubic yards | With 10% waste |
|---|---|---|
| 8 x 8 ft | 0.79 | 0.87 |
| 10 x 10 ft | 1.23 | 1.36 |
| 12 x 12 ft | 1.78 | 1.96 |
| 20 x 20 ft | 4.94 | 5.43 |
Bags or a Truck?
Mixing by hand is practical up to roughly half a cubic yard to one cubic yard, or about 20 to 40 bags. Beyond that, the labor of mixing is large and a slab poured in several batches can develop cold joints, weak lines where fresh concrete meets partly set concrete.
Ready-mix trucks charge by the cubic yard, often with a minimum load fee for small orders, typically for under 3 to 4 yards. For a 1.4-yard job, you might compare the truck fee with the cost of 82 bags and a rented mixer. Many suppliers also offer mini-mixer trucks for smaller pours.
Thickness and Preparation
A patio or walkway slab is usually 4 inches thick. Driveways and garage floors are often 5 to 6 inches thick to carry vehicles. Thickening the edges or adding reinforcement such as wire mesh or rebar helps control cracking.
A compacted gravel base of 4 inches helps drainage and prevents settling. If you add a gravel base, remember that it reduces the concrete depth only if your excavation depth is fixed, so measure the final pour depth, not the hole depth.
Ordering Tips
Round up to the next quarter yard when ordering ready-mix, because running short mid-pour is costly. Have forms, tools, and helpers ready before the truck arrives, as you usually have a limited unloading window, often 30 to 60 minutes included in the price.
Concrete strength is specified in PSI. For residential slabs 3,000 to 4,000 PSI is common.
Frequently Asked Questions
How many cubic yards of concrete do I need for a 10x10 slab?
For a 4-inch slab, 1.23 cubic yards, or about 1.36 yards with 10% extra.
How many 60 lb bags make a cubic yard?
About 60 bags (27 ÷ 0.45). An 80 lb bag yields about 0.6 cubic feet, so around 45 bags per cubic yard.
How much extra concrete should I order?
Order about 5% to 10% more than the calculated volume to cover uneven ground and spillage.
When is ready-mix cheaper than bags?
Typically above 1 to 2 cubic yards, once labor, rental, and bag costs are counted, although small-load fees can change the balance.