Fence post concrete calculator
Enter your hole size and post size. The calculator subtracts the post itself, so you don’t overbuy.
Fast-setting mix
25 50 lb bags
- Standard mix instead
- 16 80 lb bags
- Per post
- 2.46 50 lb fast-setting bags
- Total concrete
- 9.21 cubic feet
How it works
Each hole is a cylinder: π × radius² × depth. The calculator subtracts the space the post takes up and any gravel you put in the bottom, then converts cubic inches to cubic feet (1,728 cubic inches per cubic foot) and multiplies by the number of posts.
A 50 lb bag of fast-setting post mix yields about 0.375 cubic feet. A standard 80 lb bag of concrete mix yields about 0.6.
Example
With number of posts 10, hole diameter 10 in, hole depth 24 in, gravel in the bottom of the hole 0 in, post size: 4×4 (3½ × 3½ in), the answer is 25 50 lb bags.
Tips before you buy
- A common rule is to bury about a third of the post’s total length. In cold climates, footings also need to go below the frost line, so check your local code.
- A few inches of gravel in the bottom of the hole helps water drain away from the end of the post.
- Slope the top of the concrete away from the post so rainwater doesn’t pool against the wood.
- Buy one or two extra bags. Holes dug by hand always come out a little wider than planned.
Common questions
How many bags of concrete per fence post?
For a 4×4 post in a 10-inch hole 24 inches deep, about 2½ bags of 50 lb fast-setting mix, or 1½ bags of 80 lb standard mix.
How big should a fence post hole be?
Roughly three times the post’s width: about 10–12 inches for a 4×4 and 16–18 inches for a 6×6.
Fast-setting or regular concrete for posts?
Fast-setting mix is poured dry and soaked in place, and posts can usually take weight in a few hours. Regular mix is cheaper per cubic foot but you have to mix it first and wait longer.