Whether you’re pouring a new driveway, building a patio, laying a foundation, or constructing a footing, calculating the correct amount of concrete is critical. Ordering too little concrete can delay your project and increase costs, while ordering too much can lead to unnecessary waste and expense. In this detailed guide, we’ll explain how to calculate yards of concrete, including the formulas, tools, tips, and considerations to ensure you get your calculations right every time.
What Is a Yard of Concrete?
A yard of concrete refers to a cubic yard, which is a unit of volume commonly used in construction. One cubic yard equals:
- 27 cubic feet
- 3 feet x 3 feet x 3 feet
Concrete is sold by the cubic yard, so understanding how to calculate volume in cubic yards is essential when planning a project.
Why Accurate Concrete Calculation Matters
Getting the correct amount of concrete is crucial for:
- Project Efficiency: Running short on concrete means delays and labor downtime.
- Cost Control: Over-ordering concrete can be expensive and wasteful.
- Structural Integrity: Inconsistent pours can cause weak spots or joints.
In short, correct calculations = better project outcomes.
Basic Formula for Calculating Concrete Volume
The general formula for volume is:
mathematicaCopyEditVolume = Length × Width × Height (or Depth)
All dimensions must be in feet, and the result will be in cubic feet. To convert to cubic yards, divide the result by 27 (since 1 cubic yard = 27 cubic feet).
Formula in Cubic Yards:
mathematicaCopyEditCubic Yards = (Length × Width × Depth) ÷ 27
How to Convert Volume to Cubic Yards
Let’s say you have a slab that is 10 feet long, 10 feet wide, and 0.5 feet deep (6 inches).
Step-by-step:
- Multiply the dimensions:
10 × 10 × 0.5 = 50 cubic feet - Convert to cubic yards:
50 ÷ 27 ≈ 1.85 cubic yards
So, you’d need approximately 1.85 cubic yards of concrete.
Concrete Volume Calculations by Shape
Different shapes require specific formulas. Here’s how to calculate volume for the most common concrete structures:
A. Rectangular Slabs
Used for: driveways, patios, floors
Formula:Length × Width × Depth ÷ 27
Example:
For a 12′ × 20′ driveway that’s 6″ thick (0.5′):
12 × 20 × 0.5 ÷ 27 = 4.44 cubic yards
B. Cylindrical Columns
Used for: pillars, posts, piers
Formula:π × Radius² × Height ÷ 27
(Use 3.14 for π)
Example:
A column 1 foot in diameter (0.5′ radius) and 8 feet high:
3.14 × 0.5² × 8 ÷ 27 = 0.23 cubic yards
Multiply this by the number of columns.
C. Footings
Used for: foundation bases
Formula:Length × Width × Depth ÷ 27
Just like slabs, but typically deeper and narrower.
Example:
100 feet of footing, 1.5 feet wide, 1 foot deep:
100 × 1.5 × 1 ÷ 27 = 5.56 cubic yards
D. Stairs and Steps
Break stairs into individual step volumes or use an average step formula:
Formula:Number of Steps × (Step Width × Tread Depth × Riser Height) ÷ 27
Example:
3 steps, each 4’ wide, 1’ deep, and 0.5’ high:
3 × (4 × 1 × 0.5) ÷ 27 = 0.22 cubic yards
E. Odd Shapes
For trapezoids, triangular forms, or custom features:
- Break the area into simple shapes.
- Calculate each part.
- Add volumes together.
- Convert to cubic yards.
Accounting for Waste and Overages
Always order a little extra concrete to account for:
- Spillage
- Over-excavation
- Irregular ground
- Human error
Rule of Thumb:
Add 10% extra to your calculated volume.
Example:
Calculated volume: 5 cubic yards
With waste: 5 × 1.10 = 5.5 cubic yards
Most suppliers only sell full or quarter yards, so you may round up to 5.5 or 6 yards.
Using Online Concrete Calculators
Several online tools make it easier to calculate:
- Enter your dimensions
- Choose the shape
- Get the volume automatically
Popular tools include:
Still, it’s essential to understand the underlying math, especially for complex or multi-part pours.
Concrete Mixing Considerations (Ready-Mix vs. On-Site)
How you calculate may depend on how the concrete is delivered:
Ready-Mix Concrete:
- Sold by the cubic yard.
- Delivered in a truck.
- Ideal for large pours.
Order full yard increments with waste factored in.
On-Site Mixing:
- You measure by bags (e.g., 80-lb bags).
- Each 80-lb bag makes about 0.6 cubic feet of concrete.
Formula:
Cubic Yards × 27 ÷ 0.6 = Number of Bags
Example: For 1 yard:1 × 27 ÷ 0.6 ≈ 45 bags
Common Mistakes to Avoid
- Not converting inches to feet: Always divide inches by 12.
- Ignoring waste: This can leave you short during a pour.
- Rounding down: Always round up when ordering.
- Assuming bags = yards: Bagged concrete is for small jobs.
- Wrong shape formula: Use the right equation for the shape you’re pouring.
FAQs About Concrete Calculations
Can I use inches instead of feet in the formula?
Yes, but then divide the result by 46,656 (the number of cubic inches in a yard).
How thick should my concrete slab be?
- Sidewalk: 4″
- Driveway: 6″
- Garage floor: 4-6″
- Foundation: 8″+
What if the ground is not level?
Take the average depth of the highest and lowest point.
How much does 1 cubic yard of concrete weigh?
Roughly 4,000 lbs (2 tons).
How many wheelbarrows in 1 cubic yard?
Typically 13 to 14 wheelbarrows (with 6 cu. ft. wheelbarrow).