How to Calculate Concrete Needed for Slabs, Footings, and Posts

How to Calculate How Much Concrete You Need

Concrete is typically ordered in cubic yards for large pours or mixed in bags for small projects. Calculating the right amount prevents the cost of ordering extra or the hassle of coming up short mid-pour.

The Basic Formula

Volume (cubic yards) = (Length × Width × Depth) / 27

(27 cubic feet = 1 cubic yard)

Rectangular Slab Example

Patio: 12 ft long × 10 ft wide × 4 inches (0.333 ft) deep

Volume = (12 × 10 × 0.333) / 27 = 39.96 / 27 = 1.48 cubic yards

Always add 5–10% for waste: 1.48 × 1.08 = 1.60 cubic yards → order 1.75 yards

Concrete Depth Recommendations

Circular Slab

Volume = π × r² × Depth / 27

Round pad, 6 ft diameter (r = 3 ft), 4 in deep: Volume = 3.14159 × 9 × 0.333 / 27 = 9.42 / 27 = 0.35 cubic yards

Post Holes

Volume per hole = π × r² × Depth / 27

Post hole, 10 in diameter (r = 0.417 ft), 3 ft deep: Volume = 3.14159 × 0.174 × 3 / 27 = 0.061 cubic yards per hole

For 10 holes: 0.061 × 10 = 0.61 cubic yards

Bags vs. Ready-Mix

• Under 1 cubic yard: Use bagged concrete mix (80-lb bag = 0.60 cubic feet)
• 1 cubic yard+: Order ready-mix delivery (typical minimum is 1 yard)

Use our concrete calculator for any shape and depth.