The formula: volume first, bags second
Every concrete estimate starts as a volume problem. A slab, footing, or wall is a rectangular prism: volume = length × width × thickness. A column, sonotube, or fence post hole is a cylinder: volume = π × radius² × depth, where radius is half the diameter. Get the volume in cubic feet, then divide by 27 (a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet) to get the number a ready-mix plant will quote you.
The part people get wrong is thickness. Slab thickness is almost always specified in inches (4 in for a patio, 6 in for a driveway) while the footprint is measured in feet, and mixing the two without converting is the single most common way to over- or under-order by a factor of 12.
Worked example: a 10 × 10 ft patio slab, 4 in thick
Convert 4 in to feet first: 4 ÷ 12 = 0.3333 ft. Volume = 10 × 10 × 0.3333 = 33.33 cubic feet. Divide by 27: 33.33 ÷ 27 = 1.235 cubic yards. Add a 10% waste allowance for an uneven sub-base and form leakage: 33.33 × 1.10 = 36.67 ft³, or about 1.36 cu yd. That is the number to hand a ready-mix dispatcher.
For bags instead: 36.67 ft³ ÷ 0.60 ft³ per 80 lb bag = 61.1, rounded up to 62 bags (4,960 lb of mix). At 60 lb bags it is 36.67 ÷ 0.45 = 81.5, rounded up to 82 bags: same concrete, more bags to carry, because each 60 lb bag holds less mix per pound than an 80 lb bag of the same product.
Bags vs. ready-mix: where the line actually is
Bagged mix is fine for anything a couple of people can comfortably wheelbarrow-mix in an afternoon: fence posts, small footings, patch repairs, slabs under roughly a cubic yard. Past that, most contractors switch to a ready-mix truck: a full load, delivered and poured in minutes, is more uniform than dozens of hand-mixed batches, and the total labor of opening, mixing, and hauling 60-plus bags usually costs more than the delivery minimum on a ready-mix order.
Ready-mix trucks are quoted and billed in cubic yards, and most plants round an order up to the nearest quarter or half yard rather than delivering a fractional amount. So the cubic-yard figure from this calculator (with waste included) is exactly what to give the dispatcher.
Setting a fence post: the 3×-diameter rule
QUIKRETE's own installation instructions for setting posts specify digging the hole about 3 times the diameter of the post, with a hole depth equal to 1/3 of the post's overall height. Tamp roughly 6 inches of dry mix into the bottom before setting the post plumb, then pour the rest wet and let it cure at least 24 hours before the post takes any load.
That 3×-diameter rule is what the round-shape mode above uses: enter the hole diameter (not the post diameter) and the hole depth, and it returns the volume for the concrete collar around the post, not the post itself.
Why add a waste allowance at all
The math above gives the geometric volume of the finished pour, not what actually leaves the mixer. Sub-grade is rarely perfectly flat, forms flex and leak a little at the seams, and wheelbarrow spillage is real, so a 5-10% overage is standard estimating practice, higher for irregular shapes like post holes dug by hand and lower for a clean rectangular slab poured against tight forms. Order short and a job stalls waiting on a second batch or delivery; order a bit long and the leftover fills a few paver joints or a birdbath mold.