Why Concrete is Measured in Cubic Yards
In the construction industry, ready-mix concrete delivered by mixer trucks is ordered in cubic yards ($ ext{yd}^3$).
Understanding the conversion is essential:
Because slab dimensions are usually measured in feet for length and width, but inches for thickness/depth, converting dimensions properly before dividing by 27 is the most common pitfall.
Formula 1: Rectangular Slabs, Driveways & Sidewalks
Mathematical Equation
Volume (Cubic Feet) = Length (ft) ร Width (ft) ร โค ft((Depth (inches) / 12))
Mathematical Equation
Cubic Yards = (Volume in Cubic Feet / 27)
Combined Master Formula:
Mathematical Equation
Cubic Yards = (Length (ft) ร Width (ft) ร Thickness (in) / 324)
*(Note: 27 ร 12 = 324, allowing you to multiply length, width, and thickness directly and divide by 324).*
Worked Example:
Project: A patio slab $24\text{ ft long} \times 12\text{ ft wide} \times 4\text{ inches deep}$:
1
Apply the formula: (24 ร 12 ร 4 / 324) = (1,152 / 324) โ 3.555 cubic yards
2
Add 10% waste margin: 3.555 ร 1.10 = 3.91 cubic yards
3
Order: $4.0\text{ cubic yards}$ from the supplier.
Formula 2: Cylindrical Piers & Sonotube Footings
For round post holes, deck piers, and column footings:
Mathematical Equation
Volume (Cubic Feet) = ฯ ร โค ft((Radius (in) / 12))^2 ร Depth (ft)
Mathematical Equation
Cubic Yards = (Volume in Cubic Feet / 27)
Worked Example:
Project: Six (6) deck piers, each $12\text{ inches in diameter}$ (6 in radius) and $4\text{ feet deep}$:
1
Volume per pier: ฯ ร (0.5 ft)^2 ร 4 ft = 3.1416 ร 0.25 ร 4 = 3.14 cu ft
2
Total for 6 piers: 6 ร 3.1416 = 18.85 cu ft
3
Convert to cubic yards: 18.85 / 27 = 0.70 cubic yards
Estimating Pre-Mixed Bags (60 lb vs 80 lb)
When mixing by hand for smaller projects:
