Fence Post Concrete Calculator

How much concrete do fence posts really need?

The right amount of concrete for a fence post is a small volume problem: the hole is a cylinder, the post is a smaller cylinder inside it, and the concrete fills the difference. Get the diameters and depth right and the bag count falls out. This calculator does the geometry, subtracts the post's displacement (which is real, especially for 6x6 posts), and shows the total for the three most common bag sizes.

Sizing the hole

A common rule of thumb is a hole three times the post diameter, one third the post height, and always below the local frost line. That usually means a 10 to 12 inch wide hole, 24 to 36 inches deep for a residential wood fence. Corner and gate posts go bigger and deeper because they take more load.

Which bag size?

Eighty pound bags cover more per bag and are the cheapest per cubic foot, but they are heavy. Sixty pound bags are the most common choice for a home project. Fifty pound fast-setting post mix (Sakrete, Quikrete, similar) is popular because it sets in about an hour and eliminates a lot of bracing.

Ordering

Buy a couple of extra bags. If you finish under, you have a partial hole and a very short set time to get to the store. If you finish over, unopened bags of concrete keep in a dry place for months. The waste allowance in the calculator (default 10 percent) covers spillage and the slightly uneven walls of an auger hole.