Concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet and weighs roughly 4,050 pounds. A standard ready-mix truck carries 8 to 10 cubic yards. Getting the volume right matters because ordering too little means a cold joint while you wait for a short load, and ordering too much means paying for concrete you dump.
The math itself is simple: length times width times depth equals volume. The complications come from irregular shapes, thickness variations across uneven subgrade, waste in forms and footings, and the difference between theoretical volume and what you actually need on site.
Slab and Flatwork Volume Calculations
The basic slab formula is Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = Cubic Yards. A 24-by-24-foot slab at 4 inches thick is 24 × 24 × 0.333 / 27 = 7.1 cubic yards. At 6 inches thick, the same slab is 10.7 cubic yards.
Real slabs rarely hit exact theoretical volume. Subgrade is never perfectly flat. Low spots fill with extra concrete. A slab poured on undisturbed soil with good forms typically uses 5% to 8% more concrete than calculated. A slab on rough fill uses 10% to 15% more.
For monolithic slabs with thickened edges, calculate the main slab and the thickened perimeter separately, then add the volumes.
Cubic Yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27
Quick shortcut: For 4" slab, multiply square footage by 0.0123
For 6" slab, multiply square footage by 0.0185
Concrete Volume Calculator
Calculate how many cubic yards of concrete to order. Accounts for overdig, pour-specific waste factors, and short load fees. Supports slabs, footings, walls, columns, and steps.
Footing and Column Calculations
Continuous footings are rectangular in cross section. A typical residential footing is 20 inches wide by 8 inches deep. For 150 linear feet: 150 × (20/12) × (8/12) / 27 = 6.2 cubic yards.
Pier footings for pole barns are typically 24-inch or 30-inch diameter holes 48 inches deep. A 24-inch diameter pier at 48 inches deep is π × (1)² × 4 / 27 = 0.47 cubic yards each.
Round columns and sonotubes use the cylinder formula: π × radius² × height. Always round up to the nearest half yard when ordering for small pours.
Cubic Yards = π × (Diameter in ft / 2)² × Depth in ft ÷ 27
Quick reference: 24" × 48" pier = 0.47 yd³ each
12" × 10' sonotube = 0.29 yd³ each
Waste Factors and Ordering Strategy
Add waste factor to every concrete order. Slabs on smooth grade: add 5% to 8%. Slabs on rough fill: add 10% to 15%. Machine-dug footings: add 10% to 20%. Walls and formed work: add 3% to 5%. Pump pours: add 2% for line waste.
The cost of under-ordering far exceeds the cost of over-ordering. A short load delivery runs $200 to $400 minimum and creates a cold joint. Over-ordering by one yard costs $150 to $200 in extra material — cheap insurance.
Standard practice is to have a washout area or forms for a small pad where excess concrete can be placed rather than wasted.
Slab on smooth grade: +5% to 8%
Slab on rough fill: +10% to 15%
Machine-dug footings: +10% to 20%
Formed walls: +3% to 5%
Pump pours: add 2% for line waste
When in doubt, round up to the next full yard.