Post Hole Concrete Calculator

A post hole concrete calculator tells you how many bags of fast-setting concrete to buy to set fence or deck posts. Enter the hole size, how deep it is, the post width and how many posts, and it returns the bag count — including how many bags per hole so you can load the truck correctly.

Your measurements

Result

The key detail most calculators miss: the post itself takes up space in the hole, so the concrete is the hole volume minus the post volume. We subtract the post so you do not over-buy.

How it is calculated

Concrete per hole is the cylindrical hole volume (π·r²·depth) minus the square post volume over the buried depth. We total across the posts and convert to 50 lb fast-set bags at about 0.375 ft³ each, the usual yield for post-setting products.

Steps

Enter the hole diameter — a post-hole digger usually makes a 9–12 inch hole.
Enter the hole depth in inches; aim for about a third of the post height.
Enter the post width (a 4×4 is 4 inches).
Enter how many posts you are setting.
Buy the fast-set bag count shown — the "bags per post" line helps you split it.

Worked example

Setting four 4×4 posts in 10-inch holes 24 inches deep needs about 0.87 cubic feet of concrete per hole — roughly 3 50 lb bags of fast-set per post, or about 10 bags for all four.

Common questions

How deep should a fence post go?

A common rule is a third of the above-ground height, and below the frost line. A 6 ft fence post often sits in a 2 ft hole.

Do I need to mix the concrete for posts?

Fast-setting post mix can be poured dry into the hole and soaked, or mixed first. The calculator counts bags either way.

How wide should the hole be?

About three times the post width — roughly a 9–12 inch hole for a 4×4 post, which is what the default assumes.

Why subtract the post from the hole?

Because the post fills part of the hole. Subtracting it stops you from buying concrete you cannot fit.