Ordering ready-mix concrete for a DIY patio, sidewalk, or house footing requires precise volume math. If you under-order concrete, you end up with a ruined pour and cold joints. If you over-order significantly, you waste money and face expensive disposal fees.

The Core Cubic Yard Formula

Ready-mix concrete in North America is sold by the cubic yard (a 3ft × 3ft × 3ft cube equaling 27 cubic feet). Since project dimensions are usually measured in feet and inches, follow this conversion formula:

  1. Convert thickness from inches to feet by dividing by 12 (e.g. 4 inches = 0.333 feet).
  2. Multiply Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet.
  3. Divide Cubic Feet by 27 = Cubic Yards.

Step-by-Step Practical Example

Project: Pouring a backyard concrete patio slab measuring 20 feet long, 12 feet wide, and 4 inches thick:

  1. Convert thickness: 4 in / 12 = 0.333 ft.
  2. Calculate volume in cubic feet: 20 × 12 × 0.333 = 80 cubic feet.
  3. Convert to cubic yards: 80 / 27 = 2.96 cubic yards.
  4. Add 10% Waste Buffer: Subgrade unevenness and form flex require extra material. 2.96 × 1.10 = 3.25 cubic yards to order.

Calculate your material needs instantly using our Concrete Calculator.

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About the Author: David Chen, M.Sc.

Applied Mathematician & Data Specialist

David Chen holds a Master of Science in Applied Mathematics from Johns Hopkins University. He focuses on statistical analysis, algorithm optimization, educational grading models, and practical utility calculations.

Editorial Policy & Fact-Checking: Our articles are written and reviewed by certified financial planners, exercise physiologists, and mathematicians to ensure complete mathematical precision and factual accuracy.