Don't Crack Under Pressure: The Ultimate Concrete Guide
Pouring a driveway or patio? The math is simple, but the mistakes are expensive. Learn how to measure, estimate, and order like a pro.
Plan Your Pour
Don't guess. Determine exactly how many bags or truck yards you need.
Open Concrete CalculatorUnderstanding the Units: What is a Cubic Yard?
In the US, concrete is sold by the Cubic Yard (often just called a "Yard").
It is a measure of volume: 3 feet long x 3 feet wide x 3 feet high.
1 Cubic Yard = 27 Cubic Feet.
The challenge is that you measure your project in feet (Length/Width) and inches (Depth). Mixing these units without converting them first is the #1 reason for calculation errors.
The Golden Formula
To find Cubic Yards manually:
- Convert Depth to Feet: Divide inches by 12. (e.g., 4" = 0.33 feet).
- Calculate Cubic Feet: Multiply Length × Width × Depth (all in feet).
- Convert to Cubic Yards: Divide Cubic Feet by 27.
Example: 12x12 Patio at 4 Inches Thick
- Length: 12 feet
- Width: 12 feet
- Depth: 4 inches = 4 / 12 = 0.333 feet
- Volume (Ft³): 12 x 12 x 0.333 = 48 cubic feet
- Volume (Yards): 48 / 27 = 1.78 cubic yards
Slab Thickness Guide
How thick should you pour? Skimping on thickness saves money now but guarantees cracks later.
| Project Type | Thickness | Notes |
|---|---|---|
| Sidewalk / Garden Path | 3 - 4 inches | Standard for foot traffic. |
| Patio / Shed Base | 4 inches | Can support light structures and outdoor furniture. |
| Driveway (Cars) | 5 - 6 inches | Use steel mesh reinforcement strictly. |
| Driveway (RV/Heavy Truck) | 6 - 8 inches | Requires rebar grid for structural integrity. |
Bags vs. Ready-Mix Truck
This is the big decision. Do you mix it yourself or call a truck? The "cutoff" is usually 1 to 1.5 yards.
Bagged Concrete (DIY)
Best for: < 1 Yard (Sidewalks, Post Holes)
- Cost: Cheaper for small amounts.
- Labor: extremely high. Moving 45 bags (3,600 lbs) is back-breaking.
- Consistency: Hard to get every batch perfectly matched.
Ready-Mix Truck
Best for: > 1 Yard (Driveways, Foundations)
- Cost: Higher upfront min. charge (usually $150-$200 delivery fee).
- Labor: Low. The chauffeur pours it down the chute.
- Consistency: Perfect, high-strength commercial mix.
Bag Coverage Chart
Assuming a standard 0.60 cubic feet yield per 80lb bag:
- 1 Yard (27 cu ft) = 45 Bags (80lb)
- 1/2 Yard (13.5 cu ft) = 23 Bags (80lb)
- 1/4 Yard (6.75 cu ft) = 12 Bags (80lb)
Warning: If you need more than 30 bags, rent a mixer. Do not mix 30 bags in a wheelbarrow unless you want permanent back damage.
Measuring Odd Shapes
Not every patio is a square.
- L-Shapes: Divide into two separate rectangles. Calculate them individually and add the volumes together.
- Circles (Columns/Round Patios): Use the formula π × r² × depth.
Or better yet, just treat it as a square with the width equal to the diameter to give yourself a nice safety margin. - Slope: If the ground is uneven (4" at top, 6" at bottom), use the average depth (5") for your calculation.
Conclusion
Concrete is permanent. "Measure twice, pour once" is the truest cliché in construction. Always add your 10% safety margin. It is far better to dump a small pile of leftover concrete on a tarp than to frantically try to buy 3 more bags while your driveway is hardening in the sun.