How this is calculated
A slab or footing is simply length × width × depth. Post holes use the volume of a cylinder (π × radius² × depth), since a dug hole is round, not square. The bag count divides the total volume by an approximate yield of 110 bags per m³ for a standard 25kg general-purpose mix, rounded up — always check the actual yield printed on the specific bags you buy, since it varies by brand and mix.
Volume = Length × Width × Depth. A typical patio slab is 100-150mm deep — always convert depth to metres before multiplying.
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