For a rectangular slab: volume = length × width × thickness. Then add a waste factor (typically 10%) to account for spillage, uneven subgrade, and mixing losses: volume with waste = net volume × (1 + waste%). For example, a 3 m × 2 m slab 100 mm thick needs 3 × 2 × 0.10 = 0.60 m³ net, or 0.66 m³ with 10% waste. Always round up to whole bags — it is better to have a little left over than to run short mid-pour.

A standard 40 kg bag of premixed concrete yields approximately 20 litres (0.020 m³) when mixed; a 25 kg bag yields about 12.5 litres (0.0125 m³). Divide your total volume (with waste) by the bag yield and round up. For 0.66 m³: 40 kg bags needed = ⌈0.66 / 0.020⌉ = ⌈33⌉ = 33 bags. Exact yield varies by manufacturer — check your bag for the precise figure. These values are standard estimates.

The waste factor accounts for concrete lost to spillage, over-mixing, uneven subgrade (the ground is rarely perfectly level), and the small amount left in the mixer drum. A 10% waste factor is the standard rule of thumb for most residential slabs. Use 15% for sites with uneven ground, complex shapes, or multiple small pours. Use 5% for very large, simple pours where waste is minimised by a ready-mix truck.

Bagged premix is practical for small pours up to about 0.5–1.0 m³ (roughly 25–50 bags).

For larger volumes, ready-mix concrete delivered by truck is more economical and consistent — most suppliers have a minimum order of 0.2–0.5 m³ and charge a short-load surcharge below their minimum. Ready-mix is also better for structural applications (footings, foundations) where batch-to-batch consistency matters. Switch the volume unit to cubic yards (yd³) when ordering from US ready-mix suppliers.