Concrete volume is simple arithmetic. The decision that actually costs money is bags versus ready-mix, and the crossover is lower than most people think — usually somewhere around half a cubic yard.
Bag yields are fixed
An 80 pound bag of concrete mix yields about 0.60 cubic feet. A 60 pound bag yields 0.45, and a 40 pound bag yields 0.30. Those numbers are printed on the bag and do not vary much between brands.
Since a cubic yard is 27 cubic feet, one yard takes 45 of the 80 pound bags. That is 3,600 pounds of material to carry, open and mix, usually in a wheelbarrow or a rented mixer.
When to stop buying bags
Under about a quarter yard — post holes, a small pad, equipment feet — bags are clearly right.
Between a quarter and half a yard it depends on whether you can mix fast enough to keep the pour monolithic. Concrete that sets in stages leaves cold joints.
Past half a yard, call for ready-mix. Most suppliers have a minimum order and a short-load fee, but it is still usually cheaper than 25 bags once you count your time, and the result is one continuous pour.
Thickness
Four inches is standard for a patio, walkway or shed floor. Driveways and anything carrying vehicles go to five or six inches, often with reinforcement.
Thickness drives volume directly — going from four to six inches on a 10 by 20 foot slab adds nearly 1.3 cubic yards.
Order extra, and have somewhere to put it
Forms bulge, subgrade is never perfectly level, and running out mid-pour is far worse than having too much. Ten percent over is standard.
Have a form ready for the leftovers — a spare pad, a few post holes — because once the truck is there the extra is already paid for.
Control joints, and where they go
Concrete cracks. That is not a failure, it is what concrete does as it shrinks during curing, and the only real decision is whether you choose where.
Control joints are cut or tooled to about a quarter of the slab depth, which weakens the section enough that the crack forms there instead of somewhere random.
The usual spacing guide is in feet no more than two to three times the slab thickness in inches — so a four inch slab gets joints every eight to twelve feet. Panels should be roughly square; long thin panels crack across the middle regardless.
Curing is not the same as drying
Concrete gains strength through a chemical reaction that needs water. A slab that dries out in the first week stops gaining strength permanently, and it can end up substantially weaker at the surface than the mix was capable of.
Keeping it wet, covering it with plastic, or spraying a curing compound for the first several days is the difference. In hot or windy weather this matters enormously and it is the step most often skipped on a do-it-yourself pour.
The surface is the part that suffers. A poorly cured slab dusts, scales and wears at the top long before anything structural happens.