Calculate how much concrete you need in cubic yards, cubic feet, and bags for slabs, footings, columns, and walls.
Calculate how much concrete you need in cubic yards, cubic feet, and bags for slabs, footings, columns, and walls.
Measure length, width, and slab thickness. Convert thickness to feet, multiply all three, then divide by 27 for cubic yards of concrete.
Use the calculator above for instant results, then use this formula to check the math or explain the estimate.
Step-by-step instructions for estimating concrete for slabs, footings, and columns.
Concrete is ordered by the cubic yard. One cubic yard equals 27 cubic feet. For a standard 4-inch slab, one cubic yard covers about 81 square feet. For a 6-inch slab, coverage drops to about 54 square feet per yard.
Slab thickness depends on the application. Patios and sidewalks need 4 inches. Driveways need 5-6 inches for vehicle loads. Garage floors require 4-6 inches depending on use.
For small projects under 1 cubic yard, bags of premixed concrete work well. For larger pours, order ready-mix from a concrete plant. The break-even point is around 1 cubic yard (about 45 bags of 80 lb mix).
Quick reference for common slab dimensions at 4 inches thick, including 10% waste.
| Slab Size | Cubic Yards | 80 lb Bags |
|---|---|---|
| 10 × 10 ft | 1.4 | 23 |
| 12 × 12 ft | 2.0 | 33 |
| 15 × 15 ft | 3.1 | 52 |
| 20 × 20 ft | 5.4 | 91 |
| 24 × 24 ft | 7.9 | 131 |
| 30 × 30 ft | 12.2 | 204 |
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Quick answers for concrete estimation questions.
Multiply length (ft) × width (ft) × thickness (ft), then divide by 27 to get cubic yards. Convert inches to feet by dividing by 12. Add 10% extra for waste.
About 45 bags of 80 lb concrete mix make one cubic yard. Each 80 lb bag yields approximately 0.6 cubic feet, and 27 cubic feet make a cubic yard.
A cubic yard of concrete weighs about 4,050 pounds (approximately 2 tons). This varies slightly based on the mix design and aggregate used.
A 20×20 ft slab at 4 inches thick needs about 4.9 cubic yards of concrete. With 10% waste added, order 5.4 cubic yards.
Expertise covers cubic yard and square yard formulas, material quantity planning, unit conversions, and practical ordering checks for home and contractor projects.