Work out how much concrete you need for slabs, footings, or round posts — in cubic yards, cubic metres, or bags.
Pick a pour shape — a rectangular slab or footing, or a round column or fence-post hole — then enter its dimensions and how many identical pours you need. The calculator adds your waste factor on top and works out the number of pre-mixed bags or the cubic yardage to order from a ready-mix truck.
Bagged concrete makes sense for small jobs like fence posts, stepping stones, or a single footing — anything under roughly one cubic yard. Beyond that, a ready-mix truck is almost always cheaper and far less labour, since most suppliers have a minimum delivery of about one cubic yard anyway.
Uneven ground, spillage, and over-digging on footings all eat into your concrete order. A 10% waste factor is the standard rule of thumb for most DIY pours; irregular ground or a first-time pour may warrant 15%.
An 80lb bag yields about 0.6 cubic feet of concrete, so it takes roughly 45 bags to make one cubic yard (27 cubic feet).
A common rule is one-third to one-half of the post's above-ground height, with a minimum of 24 inches in most residential soil — check local building code for frost-line requirements in colder climates.
Most driveway and patio slabs use either rebar on a grid or welded wire mesh to control cracking and add tensile strength — thin decorative slabs like stepping stones typically don't need it.