How much concrete do I need?
Short answer
Multiply length × width × thickness in feet, then divide by 27 for cubic yards and add about 10% waste. A 10 × 10 ft slab at 4" is about 1.23 cubic yards — roughly 45 80-lb bags per yard. Past one yard, ready-mix is usually cheaper than bags.
Yards by slab size
| Slab (4") | Cubic yards | 80-lb bags |
|---|---|---|
| 8 × 8 ft | 0.79 | 40 |
| 10 × 10 ft | 1.23 | 62 |
| 10 × 12 ft | 1.48 | 74 |
| 12 × 12 ft | 1.78 | 89 |
| 12 × 16 ft | 2.37 | 118 |
| 12 × 20 ft | 2.96 | 147 |
| 16 × 20 ft | 3.95 | 196 |
| 20 × 20 ft | 4.94 | 245 |
| 20 × 24 ft | 5.93 | 294 |
| 24 × 24 ft | 7.11 | 353 |
| 24 × 30 ft | 8.89 | 440 |
| 30 × 40 ft | 14.81 | 734 |
| 40 × 60 ft | 29.63 | 1467 |
Bags per yard
| Bag size | Per yard |
|---|---|
| 80-lb | 45 |
| 60-lb | 60 |
| 50-lb | 72 |
| 40-lb | 90 |
Your exact project
Open the concrete calculatorFrequently asked questions
How much concrete do I need?
Multiply length × width × thickness in feet, then divide by 27 for cubic yards, and add about 10% waste. A 10 × 10 ft slab at 4" is about 1.23 cubic yards.
How many bags of concrete make a yard?
About 45 80-lb, 60 60-lb, or 90 40-lb bags per cubic yard.
When should I order ready-mix instead of bags?
Past roughly one cubic yard, ready-mix delivery is usually cheaper and far less work than mixing dozens of bags by hand. Below a yard, bags are more practical.