The Concrete Volume Formula
Concrete volume is calculated using a simple formula:
Volume = Length × Width × Depth
The tricky part is unit conversion. Most measurements are in feet and inches, but concrete is sold by the cubic yard. Here's the math:
- Convert depth from inches to feet (divide by 12)
- Multiply Length × Width × Depth to get cubic feet
- Divide by 27 to convert to cubic yards
For example: A 10ft × 10ft patio at 4 inches deep = 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards.
Bags vs. Ready-Mix: Which Should You Choose?
The break-even point is around 1 cubic yard:
- Under 1 yard: Buy bags (60lb or 80lb). More control, no delivery fees, work at your own pace.
- Over 1 yard: Order ready-mix delivery. Faster pour, better consistency, less physical labor.
A 60lb bag yields about 0.45 cubic feet. An 80lb bag yields about 0.6 cubic feet. For most DIY projects, 80lb bags are more cost-effective despite the extra weight.
Why You Need 10% Extra
Never order exactly what the calculator shows. Here's why concrete gets "lost":
- Uneven subgrade: Ground is never perfectly flat—low spots eat concrete
- Form bulging: Wood forms bow slightly under pressure
- Spillage: Some concrete inevitably misses the forms
- Over-excavation: Digging slightly too deep is common
Running short mid-pour creates a "cold joint"—a weak seam that cracks. The 10% buffer costs a few dollars but prevents expensive failures.
Frequently Asked Questions
How much does a cubic yard of concrete weigh?
A cubic yard of concrete weighs approximately 4,000 pounds (2 tons). This is why delivery trucks are necessary for large pours—even 2 cubic yards weighs as much as a car.
How long does concrete take to cure?
Concrete reaches 70% strength in 7 days and full strength in 28 days. You can walk on it after 24-48 hours and drive on it after 7 days, but avoid heavy loads for the full 28-day cure period.
What PSI concrete do I need?
For most residential projects: 2,500 PSI for footings, 3,000 PSI for driveways and sidewalks, 3,500-4,000 PSI for garage floors and high-traffic areas. Standard bagged concrete is typically 4,000 PSI.
Can I pour concrete in cold weather?
Avoid pouring below 40°F (4°C). Concrete that freezes before curing loses up to 50% of its strength. If you must pour in cold weather, use hot water in the mix and cover with insulated blankets.