The Basic Concrete Formula
Calculating concrete volume is straightforward once you understand the formula. For rectangular shapes:
Volume (cubic yards) = Length (ft) × Width (ft) × Depth (ft) ÷ 27
The "÷ 27" converts cubic feet to cubic yards (since 1 yard = 3 feet, and 3×3×3 = 27). This is the standard unit used when ordering ready-mix concrete.
Common Project Examples
10×10 Patio Slab (4" thick)
10 × 10 × 0.333 = 33.3 cu ft ÷ 27 = 1.23 cubic yards. You'll need about 56 bags (80 lb) or $150-$185 for ready-mix delivery. This is a small enough project to do with bags if you prefer.
20×20 Driveway (5" thick)
20 × 20 × 0.417 = 166.8 cu ft ÷ 27 = 6.18 cubic yards. This definitely requires ready-mix delivery — that's about 280 bags (80 lb), which would be impractical to mix by hand. Cost: $770-$1,080 for the concrete alone.
Fence Post Holes (10" diameter, 3' deep)
π × (0.417)² × 3 = 1.64 cu ft per hole. For 20 holes: 32.7 cu ft ÷ 27 = 1.21 cubic yards. About 55 bags (80 lb). Bags work well here since you're pouring in small batches.
Bags vs. Ready-Mix: Which Should You Choose?
The break-even point is usually around 1-1.5 cubic yards:
- Under 1 cubic yard: Use bags. Cost: $4-$6 per 80 lb bag × ~45 bags = $180-$270
- 1-2 cubic yards: Either option works. Ready-mix minimum orders apply.
- Over 2 cubic yards: Ready-mix is significantly cheaper. $125-$175/yd³ delivered vs. $300+/yd³ for bags.
How to Avoid Running Short
Nothing is worse than running out of concrete mid-pour. Here's how to prevent it:
- Add 5-10% waste factor — Our calculator includes this option.
- Measure twice — Re-measure your dimensions before ordering.
- Account for uneven subgrade — Ground is rarely perfectly flat; low spots use extra concrete.
- Round up — If you calculate 2.3 yards, order 2.5 or 3 yards.
Unit Conversion Quick Reference
- 1 yard = 3 feet = 36 inches
- 1 cubic yard = 27 cubic feet
- 1 cubic yard ≈ 4,050 pounds of concrete
- 4 inches = 0.333 feet
- 6 inches = 0.5 feet