The core formula
For any rectangular pour: Volume = length × width × thickness, with all three in the same unit. In feet, divide the result by 27 to get cubic yards — the unit US ready-mix plants use.
Convert inches to feet by dividing by 12: a 4 inch slab is 0.333 ft. So a 10 × 10 ft slab at 4 in is 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cu yd.
Round shapes
Cylinders — columns, round footings, post holes: V = π × (diameter ÷ 2)² × depth. A 12 in diameter, 4 ft deep hole is π × 0.5² × 4 = 3.14 cu ft.
Rings and tubes subtract the inner circle: V = π × [(D₁/2)² − (D₂/2)²] × height.
Steps and stairs
Solid steps are a sum of prisms. With n risers, run T, rise R, width W and a full-height top platform P: V = W × R × [T × n(n − 1)/2 + P × n]. The steps calculator shows this model with a labelled diagram.
Add waste, then convert
Multiply by 1.05–1.10 for formed slabs on level ground, and more for irregular trenches. Then convert: 1 cu yd = 27 cu ft = 0.7646 m³ = 764.6 litres.
For bagged premix, divide the total volume by the mixed yield printed on your bag — not the bag's weight.
Frequently asked questions
What is the formula for concrete yards?
Cubic yards = (length ft × width ft × thickness ft) ÷ 27. For round shapes, use π × radius² × depth ÷ 27.
How do I calculate concrete for an L-shaped area?
Split it into rectangles that don't overlap, calculate each, and add the volumes. Measuring around the outside double-counts the corner.