How to use it
- Pick the shape: a rectangle covers slabs, walls and footings; the round options cover post holes, columns and circular pads.
- Enter the dimensions. Each box has its own unit, so you can type the slab in feet and the thickness in inches.
- Set how many identical pours you have — for example 12 deck-pier holes.
- Keep a 5–10% waste allowance for uneven subgrade, spillage and form bulge.
The formula
Rectangle: volume = length × width × thickness
Circle: volume = π × (diameter ÷ 2)² × depth
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ bag yield (80 lb = 0.60 ft³, 60 lb = 0.45 ft³)
Worked example
| Shape | Slab / wall / footing (rectangle) |
| Length | 10 ft |
| Width | 10 ft |
| Thickness / depth | 4 in |
| Quantity | 1 |
| Waste / spillage allowance | 10 % |
Result
| Concrete needed | 1.36 cubic yards |
| Cubic feet | 36.7 ft³ |
| Cubic meters | 1.04 m³ |
| 80 lb bags | 62 bags |
| 60 lb bags | 82 bags |
| Approx. weight | 2.8 tons |
Tips before you order
- Ready-mix trucks carry about 10 yd³; below that most plants add a short-load fee per yard.
- A 4 in slab uses 1.23 yd³ per 100 ft². A 6 in slab uses 1.85 yd³ per 100 ft².
- For more than about one cubic yard, ready-mix is usually cheaper and faster than bags.
See typical concrete prices for every state on the concrete prices by state page.
Questions
How many bags of concrete are in a yard?
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb premix make one cubic yard (27 ft³).
How thick should a concrete slab be?
4 in is standard for patios, walkways and garage floors that carry cars. Use 5–6 in for heavier vehicles, RVs or hot tubs, and follow any engineered plans.
Should I order extra concrete?
Yes. Running short mid-pour leaves a cold joint. Most contractors order 5–10% over the calculated volume.