To determine how much concrete you need, follow these steps:
1. Measure Your Project Dimensions
- Measure the length , width , and thickness (depth) of the area where you will pour concrete.
- Use consistent units, typically feet for length and width, and convert thickness to feet (e.g., 4 inches = 0.333 feet)
2. Calculate the Volume
- Use the formula:
Volume=Length×Width×Thickness\text{Volume}=\text{Length}\times \text{Width}\times \text{Thickness}Volume=Length×Width×Thickness
- This will give you the volume in cubic feet
3. Convert Cubic Feet to Cubic Yards
- Since concrete is usually ordered by the cubic yard, convert cubic feet to cubic yards by dividing by 27 (because 1 cubic yard = 27 cubic feet):
Cubic yards=Cubic feet27\text{Cubic yards}=\frac{\text{Cubic feet}}{27}Cubic yards=27Cubic feet
- Alternatively, multiply cubic feet by 0.037 to get cubic yards
4. Add Extra for Waste
- Add about 5-10% extra to your volume to account for spillage, uneven subgrade, and slight measurement errors
5. Estimate Number of Bags (for small projects)
- If using pre-mixed concrete bags, calculate the number of bags based on volume:
- An 80 lb bag yields about 0.022 cubic yards.
- A 60 lb bag yields about 0.017 cubic yards.
- A 50 lb bag yields about 0.0139 cubic yards.
- For example, a 4-inch thick, 100 sq ft slab requires about 56 bags of 80 lb concrete
Example Calculation
For a 10 ft x 10 ft slab, 4 inches thick:
- Convert thickness: 4 in ÷ 12 = 0.333 ft
- Volume: 10 x 10 x 0.333 = 33.33 cubic feet
- Convert to cubic yards: 33.33 ÷ 27 = 1.24 cubic yards
- Add 10% waste: 1.24 x 1.10 = 1.36 cubic yards needed
Summary
- Measure length, width, and thickness (in feet).
- Calculate volume in cubic feet (L × W × H).
- Convert to cubic yards (divide by 27).
- Add 5-10% extra for waste.
- For bagged concrete, divide total cubic yards by yield per bag.
This method ensures you order the right amount of concrete for your project, avoiding shortages or excessive leftover material
