How the concrete calculator works
Concrete is ordered by volume. The calculator multiplies your dimensions to get cubic feet, divides by 27 to get cubic yards, then divides by how much one bag yields once mixed.
- Slab or footing: length × width × thickness. Thickness is in inches, so it is divided by 12 first. A 10 × 10 ft slab 4" thick is 10 × 10 × 0.333 = 33.3 cu ft.
- Round column or post hole: π × radius² × height. A 12" sonotube 4 ft tall holds 3.14 cu ft.
How much does one bag of concrete make?
These are the yields Quikrete publishes for its standard Concrete Mix. Other brands are close, but check the bag.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.3 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.6 cu ft | 45 |
| 90 lb | 0.675 cu ft | 40 |
Common slab sizes
| Slab (4" thick) | Cubic yards | 80 lb bags |
|---|---|---|
| 4 × 4 ft | 0.20 | 9 |
| 8 × 8 ft | 0.79 | 36 |
| 10 × 10 ft | 1.23 | 56 |
| 10 × 12 ft | 1.48 | 67 |
| 12 × 12 ft | 1.78 | 80 |
| 10 × 20 ft | 2.47 | 112 |
| 20 × 20 ft | 4.94 | 223 |
Figures without waste. Add 5–10% before you buy.
Tips for a successful pour
- Compact 4" of gravel base under slabs so the ground doesn't settle.
- Use the water amount printed on the bag. Extra water makes mixing easier but leaves weaker, crack-prone concrete.
- Cut control joints about a quarter of the slab's depth, spaced 2–3 times the slab thickness, read as feet (every 8–12 ft for a 4" slab).
- Keep the surface damp for the first days. Concrete gains strength by curing, not by drying out.
