Enter the size and number of post holes or footings you're pouring, pick your bag size, and this works out the volume and how many bags to buy — using the yield figures printed right on the bag, not a rough guess.
A round hole's volume is π × radius² × depth; a rectangular footing is just length × width × depth. Both get converted to cubic feet, multiplied by however many holes or footings you're pouring, and divided by the bag's yield to get a bag count, rounded up since you can't buy a fraction of a bag.
For anything larger than roughly half a cubic yard (about 40–45 bags of 60 lb mix), ready-mix delivery is usually both cheaper and a lot less physical labor than mixing that many bags by hand — this calculator will still give you the right number, but it's worth pricing out a small ready-mix load past that point.