Concrete Calculator
Calculate how much concrete you need for rectangular slabs and footings in cubic yards and bags, with an optional waste allowance.
Use calculatorQuestions & answers
Try an example — choose a starting point, then adjust the inputs.
How to Calculate Concrete
Cubic Yards = Cubic Feet ÷ 27
Bags = Cubic Feet ÷ 0.6 (per 80lb bag)
Example: a 10 × 10 slab at 4 inches
Convert depth to feet: 4 ÷ 12 = 0.3333 ft. Volume is 10 × 10 × 0.3333 = 33.33 ft³, or 1.235 yd³. Adding 10% gives 36.67 ft³, or 1.358 yd³. At 0.60 ft³ per 80 lb bag, round 36.67 ÷ 0.60 up to 62 bags.
The result includes the allowance you enter; do not add it again. For separate rectangular sections, calculate each section and add their volumes. Round bag counts only after combining volumes. This tool does not calculate round columns or subtract voids.
Need dimensions first? Use the square footage calculator for area and the unit converter to convert measurements.
Concrete Bag Coverage
| Bag Size | Cubic Feet per Bag | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Bag yields depend on the mix. The estimates use the yields in the table; check the yield printed on your product. See QUIKRETE Concrete Mix yield data sheet.
How many bags for a 10x10, 12x12 or 20x20 slab?
These rectangular slab examples use feet for length and width, inches for thickness, and 0.60 ft³ per 80 lb bag. The last column already includes 10% extra. Thickness is an input, not a design recommendation.
| Slab and thickness | Exact yd³ | 80 lb bags, no extra | 80 lb bags + 10% |
|---|---|---|---|
| 10 × 10 ft, 4 in | 1.23 | 56 | 62 |
| 12 × 12 ft, 4 in | 1.78 | 80 | 88 |
| 20 × 20 ft, 4 in | 4.94 | 223 | 245 |
| 20 × 20 ft, 6 in | 7.41 | 334 | 367 |
Bagged concrete or ready-mix: compare the same volume
The yardage and bag counts are alternatives for the same volume, not quantities to add together. For bagged mix, multiply your selected bag count by its quoted price. The optional price field above instead uses your ready-mix price per cubic yard. Compare delivery fees and minimum orders separately; the calculator does not estimate installation labor.