How to Calculate Concrete: Volume Conversions
A practical guide to calculating concrete volume for slabs, footings, and columns, with unit conversion formulas and worked examples.
Last updated: 2025-03-15
The Basic Concrete Volume Formula
Calculating concrete volume starts with the same principle as any volume calculation: multiply the three dimensions together. For a rectangular slab or footing:
Volume = Length × Width × Thickness (Depth)
The key challenge is unit management. In the US, concrete is ordered in cubic yards. If you measure in feet and inches, you must convert to cubic feet first, then divide by 27 (since 1 cubic yard = 27 cubic feet). In metric countries, concrete is ordered in cubic meters.
Key Unit Conversions for Concrete
- 1 cubic yard = 27 cubic feet = 0.7646 cubic meters
- 1 cubic meter = 35.3147 cubic feet = 1.30795 cubic yards
- 1 cubic foot = 0.02832 cubic meters = 0.03704 cubic yards
- Inches to feet: divide inches by 12
Worked Examples
Example 1: Concrete slab (10 ft × 12 ft × 4 inches thick)
- Step 1: Convert thickness to feet: 4 inches / 12 = 0.333 feet.
- Step 2: Calculate volume in cubic feet: 10 × 12 × 0.333 = 40 cubic feet.
- Step 3: Convert to cubic yards: 40 / 27 = 1.48 cubic yards.
- Step 4: Add 10% for waste: 1.48 × 1.10 = 1.63 cubic yards.
- Result: Order approximately 1.75 cubic yards (round up to the nearest available increment).
Example 2: Footing (20 ft long × 2 ft wide × 1 ft deep)
- Step 1: Volume = 20 × 2 × 1 = 40 cubic feet.
- Step 2: Convert: 40 / 27 = 1.48 cubic yards.
- Step 3: With 10% waste: about 1.63 cubic yards.
Example 3: Round column (diameter 18 inches, height 4 feet)
- Step 1: Convert diameter to feet: 18 / 12 = 1.5 ft. Radius = 0.75 ft.
- Step 2: Volume = π × r² × height = 3.14159 × 0.75² × 4 = 7.07 cubic feet.
- Step 3: Convert: 7.07 / 27 = 0.26 cubic yards per column.
- Result: For 4 columns: 4 × 0.26 = 1.04 cubic yards, plus waste = about 1.15 cubic yards.
Use our volume converter to convert between cubic yards, cubic meters, and cubic feet instantly.
Quick Reference Table: Common Slab Sizes at 4-Inch Thickness
| Slab Size (ft) | Cubic Feet | Cubic Yards |
|---|---|---|
| 8 × 10 | 26.7 | 0.99 |
| 10 × 10 | 33.3 | 1.23 |
| 10 × 12 | 40.0 | 1.48 |
| 12 × 12 | 48.0 | 1.78 |
| 12 × 20 | 80.0 | 2.96 |
| 20 × 20 | 133.3 | 4.94 |
| 20 × 24 | 160.0 | 5.93 |
Real-World Applications
Driveways and Patios
A typical two-car driveway is about 16 × 40 feet at 4 inches thick, requiring approximately 7.9 cubic yards. A 12 × 12 patio at 4 inches requires about 1.78 cubic yards. Always round up when ordering, as running short mid-pour creates a weak cold joint.
Foundations
Foundation footings are typically 12 to 24 inches deep and 16 to 24 inches wide. Calculate each footing segment separately, then add them together. Do not forget to include pier footings for posts and columns.
Metric Projects
For metric calculations, work entirely in meters. A 3 m × 4 m slab at 10 cm (0.10 m) thick = 1.2 cubic meters. Use our length converter if you need to convert measurements between inches, feet, centimeters, and meters.
Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply length × width × depth (all in the same unit) to get volume. Convert to cubic yards by dividing cubic feet by 27. Add 5–10% extra for waste and spillage.
How many cubic feet are in a cubic yard?
There are exactly 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft = 27 cu ft). Concrete is typically ordered in cubic yards in the US.
How much does a cubic yard of concrete weigh?
A cubic yard of standard concrete weighs approximately 3,900 to 4,050 pounds (about 1,769 to 1,837 kg). The exact weight depends on the mix design and aggregate used.
How do I convert cubic meters to cubic yards for concrete?
Multiply cubic meters by 1.30795 to get cubic yards. For example, 2 cubic meters = 2 × 1.30795 = 2.616 cubic yards.