Work out the volume before you count anything
Bags are the wrong unit to start from. Start with the form or the hole you are filling. Length times width times thickness, all in feet, gives cubic feet, and a cubic yard is 27 of those.
A four foot square pad at 4 inches thick is 16 square feet times a third of a foot, which is 5.3 cubic feet. Divide by the six tenths of a cubic foot an eighty pound bag yields and you get 9 bags. That is an answer you can carry out of a store on one cart.
Run the same arithmetic on something bigger and it stops being friendly in a hurry. A 10 by 10 pad at 4 inches is 33 cubic feet, which is 56 bags. Stacked on a pallet that is more than two tons of dry mix, and every one of them gets opened, wetted and turned by somebody.
The point where bags stop making sense
There is no rule about it, but there is a practical line and most people find it in about the same place, somewhere around half a yard. Half a yard is roughly 22 eighty pound bags. That is a long morning with a mixer and a helper and still a job you can finish.
A full yard is 45 bags. Two yards is 90. At that point you are not saving time and you are working a lot harder than the job needs. A single ready-mix load holds 10 cubic yards, which is 450 bags worth of concrete, and it shows up already mixed to a spec somebody wrote down.
What a hand mixed slab looks like when it goes wrong
The trouble with bags on a slab is not the product in the bag. It is that you are pouring in batches, and batches drift.
Water drifts first. The bag prints a water amount and the first batch gets measured carefully. By the sixth batch somebody is eyeballing it, and a wetter batch carries a higher water cement ratio and less strength than the one sitting next to it. On the finished surface that reads as patches of different color and, where the water ran high, dusting, a chalky film that comes up on a shoe or a broom.
Time drifts second. If one batch has begun to set before the next lands against it, there is a cold joint running through the slab, a seam that is both visible and weak. On a small pad with one person mixing while another places, that stays manageable. On anything the size of a shed floor it does not stay manageable for long.
What bags are genuinely good for
None of this means bags are a bad product. They are the right tool for a whole category of work, and if your job is on the list below you do not need a concrete crew for it at all.
- Setting a fence post, a mailbox post or a sign post
- A small pad under an air conditioner or a generator
- Filling a post hole where a rail or a gate post lands
- Patching a step, an edge or a broken corner
- Anchoring a basketball goal or a clothesline post
Ordering instead of hauling
Once you are past a couple of yards, the question changes from how many bags to how the concrete reaches the forms. Ready mix is ordered in 10 cubic yard loads and a job rounds up to the next full load. Plants have their own arrangements for anything under a full one, and that is worth asking about directly when you call.
Whichever way the concrete arrives, add 10 percent to your calculated volume for waste. Forms flex, the subgrade is never perfectly flat and some always stays behind in the chute or the mixer. That is the reason a job that calculated out at exactly your bag count comes up two bags short with the light going.