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Concrete Calculator: Cubic Yards and Bags
Calculate how much concrete you need for a slab, patio, footing, wall, post holes or round columns, in cubic yards for a truck or in 40, 60 and 80 lb bags.
How to calculate concrete
Concrete is sold by the cubic yard, which is 27 cubic feet. For a slab, multiply length × width in feet by the thickness in feet (inches ÷ 12), then divide by 27. A 10 × 10 ft patio, 4 inches thick, is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cu yd. With 10% extra, order about 1.36 cu yd.
For a round hole or column, the volume is π × radius² × depth. A 12 in diameter hole 3 ft deep holds 3.14 × 0.5² × 3 = 2.36 cu ft, which is four 80 lb bags before any extra for waste.
How many bags of concrete do I need?
One bag of standard concrete mix yields about:
| Bag | 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 |
Common slab thicknesses
- 4 inches for patios, walkways, sheds and garage floors for cars.
- 5 to 6 inches for driveways and floors with heavier vehicles.
- Footings are often 8 to 12 inches deep and twice as wide as the wall above. Your local building code sets the minimum.
Why order extra?
Ground is never perfectly flat, forms bow slightly and some concrete is always left in the mixer or wheelbarrow. 5 to 10% extra is normal. Running short in the middle of a pour leaves a weak cold joint, which is far worse than a little left over. Many ready-mix plants charge a short-load fee for orders under 3 to 5 yards.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 in thick is 1.23 cubic yards, which is 56 bags of 80 lb concrete, or about 62 with 10% extra.
How many 80 lb bags of concrete make a yard?
45 bags. Each 80 lb bag makes about 0.6 cu ft, and a cubic yard is 27 cu ft.