How this is calculated
Concrete volume is length × width × depth, with all three dimensions converted to the same unit before multiplying. Depth is usually the smallest of the three, so it's easy to enter it in the wrong unit — this calculator keeps depth in inches (or centimetres) specifically because that's how depth is specified on a real set of plans.
Ready-mix suppliers and bagged concrete are both sold in fixed volume increments, so this calculator adds a waste allowance on top of the raw volume before recommending an order quantity — pouring exactly the calculated volume with no allowance for spillage, an uneven subgrade, or over-excavation almost always comes up short.
The calculator treats the pour as a clean geometric shape. Thickened edges, steps, piers, trenches, and sloping slabs should be measured as separate sections and added rather than hidden inside an average depth.
Order units differ by market: US plants normally quote cubic yards, while UK suppliers quote cubic metres. A supplier can also tell you whether the calculated load falls below a truck minimum or needs rounding to the nearest deliverable quantity.
Formula & methodology
Volume = Length × Width × Depth. Cubic yards = Volume (ft³) ÷ 27. Recommended order = Volume × (1 + waste %).
Worked example
A 10 ft × 10 ft patio slab, 4 in thick
- 1.Convert depth to feet: 4 in ÷ 12 = 0.33 ft
- 2.Volume = 10 ft × 10 ft × 0.33 ft = 33.3 ft³
- 3.Cubic yards = 33.3 ÷ 27 = 1.23 yd³
- 4.With a 10% waste allowance: 1.23 × 1.10 = 1.36 yd³ — round up to what your supplier can deliver.