Figuring Concrete by the Yard

[MikePhua]

Understanding Concrete Measurement
Concrete is typically ordered and measured by the cubic yard, a volumetric measurement reflecting three-dimensional space. One cubic yard equals 27 cubic feet (3 feet × 3 feet × 3 feet). This measure helps suppliers and contractors estimate how much concrete is needed to fill a specific area.
Calculating Concrete Volume
The basic formula for figuring concrete volume is:
Length × Width × Depth (or Thickness) = Volume in cubic feet
This volume in cubic feet is then converted into cubic yards by dividing by 27.
For example, if you have a slab that is 10 feet long, 5 feet wide, and 0.5 feet (6 inches) thick:

1. Multiply the dimensions: 10 × 5 × 0.5 = 25 cubic feet
   
2. Convert cubic feet to cubic yards: 25 ÷ 27 ≈ 0.93 cubic yards
   

Adjusting for Waste and Over-Ordering
It’s common practice to add about 10% extra concrete when ordering to account for spillage, uneven subgrades, and minor miscalculations. Using the above example, 0.93 cubic yards becomes approximately 1.02 cubic yards ordered.
Measurement Units and Conversion
When dimensions are in inches, convert the thickness to feet by dividing by 12. For instance, 4 inches thickness equals 4 ÷ 12 = 0.333 feet.
If the area isn’t a simple rectangle (e.g., circular columns or curved slabs), specific geometric formulas apply:

• For cylinders: Area = π × radius², then multiply by height (thickness)
  
• For irregular shapes: Divide the area into basic shapes, calculate each, then sum the volumes.
  

Concrete Pour Thickness Varieties
Standard slab thickness varies by application:

• Residential patios and sidewalks: 4 inches
  
• Garage floors or driveways: 5-6 inches
  
• Commercial and industrial slabs: 6-8 inches or thicker
  

Always measure thickness precisely, considering actual form dimensions, as nominal lumber sizes (like 2x4s) often differ from actual measurements (3.5 inches wide).
Estimating Bagged Concrete Needs
If mixing concrete from bags, calculate total volume similarly, then determine how many bags are needed based on the volume each bag yields.

• An 80-pound bag yields about 0.6 cubic feet of concrete.
  
• To find the number of bags: total cubic feet ÷ 0.6 = number of bags required.
  

Tips for Accurate Estimates

• Measure twice to avoid ordering shortages or excessive leftovers.
  
• Include a waste factor to accommodate spillage and uneven subgrade.
  
• Consult with concrete suppliers for minimum order quantities and delivery constraints.
  
• Use online calculators or apps for quick volume and bag calculations.
  

Practical Example
For pouring an 80 ft. driveway, 24 ft. wide and 5 inches thick:

1. Convert thickness to feet: 5 ÷ 12 = 0.416 ft
   
2. Calculate volume in cubic feet: 80 × 24 × 0.416 ≈ 799 ft³
   
3. Convert to cubic yards: 799 ÷ 27 ≈ 29.6 cubic yards
   
4. Add 10% for waste: 29.6 × 1.1 ≈ 32.6 cubic yards ordered
   

Terminology Explained

• Cubic Yard: A standard volume measurement for concrete, equivalent to 27 cubic feet.
  
• Slab Thickness: The vertical height of the concrete pour, critical for strength and volume.
  
• Waste Factor: An additional amount of material ordered to cover installation variances.
  
• Nominal Lumber Size: The labeled size of lumber, which differs from its actual dimensions.
  

These calculations ensure accurate ordering and efficient use of concrete, avoiding costly delays and material waste on construction projects.