A yard of concrete is 45 bags, and that changes how you order
A ten foot by ten foot patio at four inches thick does not look like a math problem. Measure it, multiply it, pour it. The part that actually costs a trip back to the yard is not the pour, it is the ordering, and the number behind it is one most owners have never sat down and worked out.
The cubic feet come first
Length times width times thickness in feet is the whole formula. For a 10 by 10 slab at 4 inches, that is 10 x 10 x (4 / 12), which is 33.3 cubic feet. Divide by 27 to get cubic yards, since a yard of concrete is 27 cubic feet by definition, and you land on 1.23 yd3. That is the slab as it actually sits once it is cured. It is not what you order.
Waste is not padding, it is the difference between one trip and two
Add a 10 percent waste allowance, which covers an uneven subgrade, spillage, and the last wheelbarrow that never quite makes it, and 33.3 cubic feet becomes 36.7. Divided by 27, that is 1.36 yd3. The gap between 1.23 and 1.36 is small on paper and large on a Saturday, because a ready-mix plant does not sell you 1.23 of anything.
Ready-mix sells in quarter yards. Round up.
That means 1.36 does not get ordered as 1.36. It gets ordered as 1.50, the next quarter yard up. The waste allowance and the rounding rule are doing two different jobs. One protects you from running short mid-pour. The other protects you from a plant telling you they cannot split a quarter yard for you on site.
What the same job looks like in bags
An 80 lb bag yields 0.6 cubic feet once mixed, and a 60 lb bag yields 0.45. That means one full cubic yard, with no waste added at all, takes 27 / 0.6, which is 45 bags of 80 lb mix, or 27 / 0.45, which is 60 bags of 60 lb. Forty five bags at 80 lb each is 3,600 lb of dry product for a single yard, before a wheelbarrow has moved.
For the actual patio, with the 10 percent waste already folded in, that is 36.7 / 0.6, rounded up, which is 62 bags of 80 lb mix. Sixty two bags at 80 lb comes to just under 2.5 tons of bagged material to replace 1.36 yards that a single truck would deliver already mixed. That is the number that usually decides whether a slab gets mixed by hand or ordered wet, Decide that before you book the truck or load the bags, not during either one..
Run your own numbers before you call it in
Put in your length, width and thickness and the concrete calculator will give you the actual yardage, the order yardage with waste built in, and both bag counts side by side, so you know which way to call it before you are standing in the aisle.