Rectangular slabs and footings, or round post holes — exact volume and the number of 20kg or 25kg bags you need to buy.
One 25kg bag of general-purpose concrete mix yields approximately 0.012 m³ of mixed concrete — about 84 bags per cubic metre. A 20kg bag scales down proportionally (~0.0096 m³/bag). Always round up and add a waste allowance — concrete settles, spills, and uneven ground eats more than the geometric minimum.
For pours under roughly 0.5 m³, bagged mix is usually the practical choice. Above that, a ready-mix delivery becomes more economical and far less physical labour — this calculator flags it automatically once your volume (with waste) crosses that threshold.
A 2.4m × 1.8m shed base at 100mm deep needs 0.432 m³ of concrete — with a 10% waste allowance, that's 40 bags of 25kg mix (1,000kg total). Six fence post holes at 300mm diameter, 600mm deep, need 0.254 m³ — 28 bags of 20kg mix with a 5% allowance.