Calculate the volume of concrete for a slab or footing and how many bags of pre-mix you need.
Volume is length × width × thickness. Bag counts assume a typical pre-mix yield of roughly 0.45 litres of set concrete per kilogram, so they are an estimate; check your product, and order a little extra. For large pours, ready-mix delivery is usually cheaper than bags.
Multiply length by width by thickness, all in the same units. For a slab 3 m by 3 m and 10 cm thick that is 3 × 3 × 0.10 = 0.9 cubic metres. This tool converts the thickness from centimetres for you.
It depends on the bag, but a rough guide is about 110 × 20 kg bags, 88 × 25 kg bags, or 55 × 40 kg bags per cubic metre of set concrete. The exact yield is on the bag.
Bags are convenient for small jobs, but mixing many by hand is hard work and inconsistent. For roughly half a cubic metre or more, a ready-mix delivery is usually easier, stronger and cheaper.