Free Concrete Bags calculator with clear step-by-step results.
Slab volume is length by width by thickness, grossed up for the sub-base irregularity and spillage that every pour loses, then divided by the mixed yield printed on the bag. Bag count is rounded up because you cannot buy a partial bag, and the mass figure is a reality check on handling.
Slab volume
Volume m3 = length x width x thickness mm / 1000
Bags required
Bags = ceil(volume x 1000 x (1 + waste) / yield litres per bag)
Around 9-10 litres of mixed concrete; a 25 kg bag gives about 12 litres and a 40 lb bag roughly 0.3 cubic feet. Check the bag, as yields differ by mix.
Above roughly 0.5 cubic metres - about forty 20 kg bags - hand mixing becomes slow enough that cold joints form between batches.