Coverage depends on thickness, not just area
Concrete volume is area multiplied by thickness, so the same cubic yard spreads over very different areas depending on how thick the slab is. One cubic yard is 27 cubic feet; dividing by the slab thickness in feet gives the area it covers. A 4-inch slab is one-third of a foot thick, so a cubic yard covers 27 ÷ 0.333 = 81 square feet.
| Slab thickness | Area covered by 1 cubic yard |
|---|---|
| 3 inches | 108 sq ft |
| 4 inches | 81 sq ft |
| 5 inches | 64.8 sq ft |
| 6 inches | 54 sq ft |
Bag yields: how many bags per cubic yard
Bagged concrete mixes state their yield on the bag. Using the common manufacturer yields, an 80-lb bag makes about 0.60 cubic feet, a 60-lb bag about 0.45 cubic feet, and a 40-lb bag about 0.30 cubic feet. Dividing 27 cubic feet by each yield gives the number of bags needed to make a full cubic yard.
| Bag size | Approx. yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Why you add a waste factor
Ordering the exact calculated volume almost always leaves you short. Concrete is lost to spillage, to an uneven or over-excavated subgrade that takes more than planned, to material stuck in the mixer or wheelbarrow, and to the simple fact that a slab is rarely a perfect rectangle of uniform depth. A waste factor of roughly 5–10% added to the calculated volume covers these losses; the rougher the subgrade and the smaller the pour, the closer to 10% is prudent.
Running out mid-pour is far more costly than a small over-order, because a cold joint forms where fresh concrete meets concrete that has already begun to set. For ready-mix deliveries in particular, ordering slightly over the calculated volume is standard practice.
A worked example
For a 10 ft × 12 ft patio slab at 4 inches thick: the area is 120 square feet, and at 81 square feet per cubic yard that is 120 ÷ 81 = 1.48 cubic yards. Adding a 10% waste factor gives about 1.63 cubic yards to order. In 80-lb bags, the base 1.48 cubic yards would need about 67 bags before waste, so roughly 74 bags with the same margin — which is why larger slabs are usually cheaper to pour with ready-mix than with bags.
常见问题
How much area does a cubic yard of concrete cover?
It depends on thickness: 81 square feet at 4 inches, 54 square feet at 6 inches. One cubic yard is 27 cubic feet, divided by the slab thickness in feet.
How many 80-lb bags of concrete make a cubic yard?
About 45 bags. An 80-lb bag yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet.
How much extra concrete should I order?
Add a waste factor of about 5–10% to your calculated volume to allow for spillage, uneven subgrade and non-uniform depth. Running short mid-pour is worse than a small over-order.
How much concrete for a 10x12 foot slab at 4 inches?
About 1.48 cubic yards before waste (120 sq ft ÷ 81), or roughly 1.63 cubic yards with a 10% waste factor.
参考文献
- Portland Cement Association (PCA) — concrete volume and cubic-yard basics. https://www.cement.org/
- QUIKRETE — concrete mix bag yields (80 lb ≈ 0.60 cu ft; 60 lb ≈ 0.45 cu ft; 40 lb ≈ 0.30 cu ft). https://www.quikrete.com/
- American Concrete Institute (ACI) — guidance on ordering and placing concrete. https://www.concrete.org/