Every Quikrete concrete yields the same per pound
This is the single most useful thing to know before you buy, and almost nobody says it. Work through QUIKRETE's data sheets for Concrete Mix (No. 1101), Fast-Setting (No. 1004), QUIKRETE 5000 (No. 1007) and Crack Resistant (No. 1006), divide every published US bag yield by its bag weight, and you get exactly the same number every time: 0.0075 cubic feet per pound. 40 lb makes 0.30. 50 lb makes 0.375. 60 lb makes 0.45. 80 lb makes 0.60. 90 lb makes 0.675.
So choosing between QUIKRETE products does not change how many bags you need for a given volume. It changes how fast it sets, how strong it gets and how it behaves as it shrinks. If you find a "Quikrete 5000 calculator" that returns a different bag count from a plain Quikrete calculator for the same slab, it is not modelling anything — it is a page with a different word in the title.
What genuinely does change with product is which bag sizes exist. Concrete Mix runs from 40 lb up to 90 lb. Fast-Setting stops at 60 lb. QUIKRETE 5000 and Crack Resistant top out at 80 lb. That is why the product selector here filters the comparison table rather than changing any arithmetic.
The number nobody puts next to the bag count
Bigger bags are better value per cubic foot, and the 90 lb bag of Concrete Mix is the cheapest concrete QUIKRETE sells by volume. Every calculator will happily tell you that you need 55 of them. Very few mention that this is 4,950 pounds of bagged material you are going to pick up twice — once into the cart and once at the mixer.
So the comparison table carries total lift weight beside every bag count, along with the spare volume you end up owning after whole-bag rounding. On a small pour that rounding sometimes erases the larger bag's advantage entirely: buying one 90 lb bag to cover a shortfall of a tenth of a cubic foot is worse value than one more 40 lb bag. The point is to let you trade money against effort with both numbers visible, rather than being handed one recommendation.
Related and worth saying plainly: past roughly a cubic yard, bagged concrete stops making sense. The calculator flags it when your volume crosses that line. Mixing 60 bags by hand is a day of work and an inconsistent pour, and a ready-mix truck will usually be cheaper as well as better.
Where Quikrete's own post example disagrees with the geometry
The Fast-Setting data sheet gives a worked example: "Two 50 lb bags of QUIKRETE Fast-Setting Concrete Mix will set a 3 in square post in a 9 in diameter hole 30 in deep." Two 50 lb bags is 0.75 cubic feet. It is the kind of figure that gets quoted everywhere without anyone checking it, so we checked it.
Using QUIKRETE's own installation method — fill to 3 to 4 inches below the top of the hole, not to grade — the concrete occupies the ring between a 9 inch circle and a 3 inch square post, 27 inches deep. That is 0.853 cubic feet. The published example is roughly 12% short of it. Not wrong exactly; it is a round, memorable number for a common case, and two bags will set that post firmly. But if you are buying for six posts and would rather not stop halfway, buy the geometry.
This calculator computes the annulus and subtracts the post, which is the step most tools skip. On a 4×4 in a 12 inch hole the post itself is over a tenth of the volume — ignore it and you buy more than you need on every single hole. Depth is a different question again: QUIKRETE suggests one third of the overall post height, but in a cold climate frost depth governs and is usually deeper. The fence post depth calculator handles that side of it.
What this page is not
It is not a general concrete volume calculator. If you want the pure geometry across slabs, footings and post holes without the QUIKRETE product line attached to it, the concrete bag calculator does exactly that and works for any brand. This page exists for the narrower question of which QUIKRETE product, in which bag, and what that costs you in trips and lifting.
It also will not size a footing, tell you how thick a slab should be, or confirm a post depth meets code. Those come from your drawings, your local building department, or an engineer. For a round column poured in a forming tube, the Sonotube concrete calculator works from actual published tube diameters, and rebar is a separate takeoff again.