Unless your soil compacts as hard as concrete, you should surround your fence posts with concrete in order to hold them firmly. But how much? What kind? Do you want concrete footings showing under each post? It doesn't matter if your fence is made of galvanised pipe, vinyl or wood -- securing your posts is the key to a long lasting installation that looks great for years.
The depth the fence post holes must be dug or augured depends on the maximum frost depth during winter. Fence post holes can be as little as two feet in the south, or six feet or more in the north. Obtain hole depth information from your local building and code department. You'll need to consult them for any necessary permits, so get information early.
Frost will cause the posts to heave up if the post footings are not below the frost line, which is the reason for sinking posts so deep in some regions. Even if you have no frost depth, you should still sink the posts to half their above-ground height. A six-foot-high post should be nine feet long, three feet of which are in the ground. For very deep footings, sometimes the post is only set part way into the hole, with the bottom of the hole filled with concrete.
Dig six-inch diameter holes for two inch galvanised pipe posts. For optimum holding power, dig eight-to twelve-inch diameter holes for 4 x 4 posts.
Different Types of Concrete
You can order ready-mix concrete, usually in one cubic yard increments. You'll need a few fence posts to use up an entire cubic yard of concrete. The driver will be in a hurry to get the concrete from his truck before it sets. Plan on bringing in friends to help and having two or more wheelbarrows to get it done quick.
You can mix it yourself in a wheel barrow or mixing trough. Ready to mix concrete is sold in 40, 50, 60, 70 and 80 pound bags. Just add water, mix thoroughly and pour. These two methods provide the strongest and longest lasting concrete.
Quick set concrete mix is poured directly into the hole, and water is added to it. In short order, the concrete sets and holds the post firmly in place. These types of mixes do a great job of holding the post in place for a number of years, before beginning to break down. They are more expensive than standard mixes, though.
Concrete Volume Facts
Each 18.1 Kilogram of unmixed concrete makes .3 cubic feet of mixed concrete.
Two-inch diameter posts set in six-inch diameter holes require one 40 pound bag for every 24 inches of depth. You'll have a little left over, which you can pour into the next hole.
4 x 4 posts set in ten-inch diameter holes require just over three 40 pound bags for every 24 inches of depth. Leaving a little space for dirt to plant grass, three bags will be just about perfect for each hole.
If you order ready mix concrete, remember that one yard will set about 30 - 4 x 4 fence posts in 10 inch diameter, two foot deep holes.
1 cubic yard = 27 cubic feet. One 4 x 4 post in a 10 inch diameter x 24 inch deep hole requires .92 cu feet of concrete to fill the hole. 27 cu feet / .92 cu feet = 29.3 holes. If you leave a little out of each hole to fill with dirt, you can fill 30 holes.
One yard of concrete requires six to eight heavy wheelbarrow loads to transport, depending on the size of the wheelbarrow.