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