Setting fence posts in concrete keeps a fence straight and strong for years, but guessing the bag count leads to either a half-finished row or a pile of wasted bags. This free fence post concrete calculator gives you the count before you go to the store.
The math is simple. First the tool finds the volume of the hole, which is a cylinder. Then it subtracts the volume of the post itself, because the post takes up space the concrete cannot fill. What is left is the concrete each post needs.
Then it divides that volume by the yield of one bag. An 80-pound bag makes about 0.6 cubic feet of concrete, a 60-pound bag about 0.45, a 50-pound bag about 0.375, and a 40-pound bag about 0.3. The tool rounds up per post, because you cannot buy part of a bag.
A common setup is a 12-inch hole, 24 inches deep, with a 4×4 post. That needs roughly two 80-pound bags per post, but try your own numbers: a wider hole or a deeper hole raises the count fast, since volume grows with the square of the diameter.
Hole depth matters for strength, not just for concrete. A good rule is to bury about one third of the post, and to go below the frost line in cold climates so winter frost does not push posts up. Check local rules before you dig, and always call your local utility marking service first so you do not hit a buried line.
This calculator gives an educational estimate only. Real quantities depend on soil, hole shape, local codes, and your supplier bag yields. Buy one or two extra bags for uneven holes, and confirm before buying or building.
Enter your hole size, post type, and post count, and take the total to the store with confidence. It is free and needs no sign-up.