How to Use the Concrete Calculator
Our concrete calculator is designed to simplify your construction planning. Whether you are pouring a new driveway, setting fence posts, or building a foundation, knowing exactly how much material to order is crucial for both your budget and the environment.
Understanding the Formulas
Concrete is measured by volume. The basic formula for a rectangular slab is simple: Volume = Length × Width × Thickness. However, because concrete is typically sold by the cubic yard in the US, or cubic meter internationally, conversions are necessary.
Cubic Feet = Length(ft) × Width(ft) × [Thickness(in) / 12]
Cubic Yards = Cubic Feet / 27
# Column Calculation (Cylindrical)
Volume = π × Radius² × Height
Practical Example: A Standard Patio
Imagine you are pouring a patio that is 12 feet long, 10 feet wide, and 4 inches thick.
- Calculate Area: 12 ft × 10 ft = 120 sq ft.
- Convert Thickness: 4 inches is 0.33 feet (4 / 12).
- Calculate Volume in Cubic Feet: 120 sq ft × 0.33 ft = 40 cubic feet.
- Convert to Cubic Yards: 40 / 27 = 1.48 cubic yards.
- Add Waste: With a 10% waste factor, you should order approximately 1.63 cubic yards.
The Importance of the Waste Factor
We recommend a standard 10% waste factor for most projects. Why? Soil is rarely perfectly level, forms may bulge slightly under the weight of wet concrete, and some material will always be left in the mixer or wheelbarrow. Running short on concrete in the middle of a pour is a stressful and costly mistake—ordering a little extra ensures a smooth finish.
Ordering Pre-Mixed vs. Bagged Concrete
For small projects (under 1 cubic yard), buying bags from a local hardware store is often the most economical choice. However, for larger projects, ordering a "ready-mix" truck is much faster and ensures a more consistent mix quality. Use our calculator to see exactly how many 80lb or 60lb bags you would need to haul—you might be surprised how heavy even a small slab can be!
Common Mistakes to Avoid
1. Measuring the exterior of forms: Always measure the interior dimensions where the concrete will actually sit.
2. Inconsistent depth: If your sub-base is uneven, measure the depth at multiple points and use the average depth for your calculation.
3. Forgetting the "Yield": A bag of concrete mix doesn't just turn into its weight in volume. An 80lb bag yields about 0.6 cubic feet of wet concrete.