Concrete Calculator
Volume of concrete for a slab, footing or column, and bags needed.
How it works
Volume is straightforward — length x width x depth, or πr²h for a column. The judgement is in the allowance.
Ten per cent is the usual figure, and it covers more than spillage: subgrade that is not as level as it looked, formwork that bows under the pour, and the fact that running short mid-pour leaves a cold joint you cannot undo. Over-ordering wastes money once; under-ordering costs a weekend.
Above roughly a cubic metre, ready-mix is almost always cheaper and always better than mixing bags by hand.
Common questions
How many bags to a cubic yard?
About 45 of the 80 lb bags, or 60 of the 60 lb. That is 45 bags to mix by hand, which is the point at which ready-mix stops being a luxury.