How much concrete per fence post?
Enter the post count, hole size and post size.
You get the bags to buy: with the volume the post itself displaces already taken off.
Worked example: 14 posts, 12 in holes, 30 in deep
A 6x6 post is 5.5 in square, and it displaces a meaningful share of a 12 in hole. Deducting it is the difference between finishing the job with bags to spare and driving back to the store.
The two mistakes that cost the most
Ignoring the post
The post displaces roughly a fifth of the hole. On a thirteen-post fence that is several bags. The calculator deducts it, which is the only reason the number here is lower than a simple cylinder calculation would give.
Mixing it too wet
Post concrete should be mixed stiff: just enough water to hold together when you squeeze a handful. A sloppy mix is weaker, shrinks as it dries and pulls away from the post, which is exactly the gap water and frost get into. Pour it dry-ish, then tamp it as you fill with a length of scrap timber, working it around the post to close voids.
Finishing the hole
Slope the top of the concrete away from the post so water runs off rather than pooling at the base. Stop the concrete about an inch above the surrounding soil so the base of the post stays dry. If you are in a very wet spot, a small gravel layer at the bottom of the hole gives water somewhere to go, though the concrete still needs to bear on undisturbed soil rather than on the gravel.
When to buy bulk instead
Bagged concrete is the practical choice up to about a pallet. Past that, the bags are heavy enough that a delivered mix or a bulk bag of ballast and a mixer works out cheaper and easier. The calculator reports the pallet count so you can see where you land.