Ordering concrete for a DIY project can be a daunting task. Order too little, and you're stuck with a "cold joint" and a frantic call to the ready-mix plant. Order too much, and you're literally throwing money down the drain. Understanding exactly how to calculate a yard of concrete is an essential skill for any homeowner or builder.
Concrete Yardage Calculator
The Fundamental Formula
Concrete is sold and measured by the cubic yard. To find out how many cubic yards you need, you first need to calculate the volume of your project in cubic feet and then convert that to cubic yards.
The standard formula for a rectangular slab is:
- 1. Multiply Length (ft) x Width (ft) x Thickness (ft) = Cubic Feet
- 2. Divide Cubic Feet by 27 = Cubic Yards
Converting Inches to Feet
Most people measure the thickness of a slab in inches (like a 4-inch driveway). To use the formula correctly, you must convert those inches into feet by dividing by 12. For example, a 4-inch slab is 4/12, or 0.33 feet thick.
Step-by-Step Calculation Example
Let's say you want to build a patio that is 12 feet long, 12 feet wide, and 4 inches thick.
- Step 1: Convert thickness to feet: 4 / 12 = 0.333 ft.
- Step 2: Multiply the dimensions: 12 x 12 x 0.333 = 47.95 cubic feet.
- Step 3: Convert to yards: 47.95 / 27 = 1.77 cubic yards.
Why the "Waste Factor" Matters
In the real world, ground is never perfectly level, and forms can bulge slightly under the weight of wet concrete. If you order exactly 1.77 yards, you will likely run short. Industry professionals always recommend adding a 10% waste factor.
Using our previous example: 1.77 yards x 1.10 = 1.95 cubic yards. You would likely order 2.0 yards from the plant to be safe.
Common Concrete Project Thicknesses
Depending on what you are building, the standard thickness varies:
- Sidewalks and Patios: 4 inches is standard for foot traffic.
- Driveways: 4 to 6 inches, depending on the weight of the vehicles (6 inches is better for heavy trucks).
- Garage Floors: 5 to 6 inches.
- Footings: Usually 8 to 12 inches deep.
Ordering Tips
When you call the concrete supplier, have your cubic yardage ready, but also be prepared to discuss the "mix design" (PSI). For most residential projects, a 3,000 to 4,000 PSI mix is sufficient. Also, ask about "short load" fees; many companies charge extra if you order less than 3 or 4 cubic yards at a time.