Formula
Thickness is entered in inches, so the cubic-foot and cubic-yard conversions collapse into one divisor:
cubic yards = length × width × thickness / 324
Dividing by 324 is the same as converting inches to feet and cubic feet to cubic yards. Waste and packaged bags stay on the concrete calculator.
Assumptions
- Length and width are feet.
- Thickness is inches.
- The slab is a flat rectangle.
Worked example
A 20 by 20 foot slab at 4 inches is 4.94 yd³.
Input definitions
- Thickness
- The wet depth in inches, not in feet.
- Cubic yard
- The unit most ready-mix plants sell. One cubic yard is 27 cubic feet.
Limitations
This is a planning quantity. It is not a structural design, a permit drawing, or a supplier's guaranteed yield.
Questions
Why is the divisor 324?
Twelve inches in a foot and 27 cubic feet in a yard multiply to 324.
Does this count bags?
No. The bag page starts from cubic yards you already know. The concrete page starts from a box measured in one unit.