Concrete comes to your project one of two ways: in 60 or 80 lb bags you mix yourself, or in a truck that backs up to your forms. The right choice is almost entirely a function of volume, and the crossover point is lower than most people think.
The one cubic yard rule
An 80 lb bag yields about 0.60 cubic feet of concrete. A cubic yard is 27 cubic feet, so a yard of concrete is 45 bags. Mixing 45 bags by hand or in a rented mixer is a full, punishing day, and the slab you pour in batches over six hours is weaker than one poured in forty minutes, because the early batches start setting before the last ones arrive.
That gives you the rule: under about one cubic yard, use bags; over it, price ready-mix. Run your dimensions through the concrete slab calculator to see which side you land on.
What bags are actually for
Bags win on small, scattered, or hard-to-reach pours:
- Fence and deck post footings. Two 60 lb bags per typical fence post hole; a 12 inch tube form 3 feet deep takes four 80 lb bags. The footing calculator counts these per hole.
- Small pads. An AC condenser pad or a 4 x 4 landing is a handful of bags.
- Repairs and setting hardware. Patching a step, setting a mailbox or basketball hoop.
Bags also solve access: a truck cannot reach a backyard footing 200 feet from the street, but a wheelbarrow can.
What ready-mix changes
Ready-mix is sold by the cubic yard, usually with a 1 yard minimum and a short-load fee below 3 or 4 yards. Even with the fee, it is usually cheaper per yard than bags, and enormously cheaper in labor. A 10 x 10 patio at 4 inches (about 1.4 yards with waste) would be 62 bags mixed by hand; a truck pours it in fifteen minutes with time to finish the surface properly.
Two things to have ready before the truck arrives: your forms braced and your crew scheduled. Concrete does not wait, and neither does the driver, since standby time is billed by the minute after a grace period.
Ordering the right amount
Whichever route you take, add 5 to 10 percent for waste: uneven subgrade, spillage, and forms that bow all eat concrete, and a short pour is a genuine problem because a cold joint between today's concrete and tomorrow's is permanently weak. Every concrete calculator on this site builds the waste factor in and shows both the yard figure for the dispatcher and the bag counts for the register.