The task of erecting fences starts with digging the holes for the wooden fence posts. However, the needs of wood posts for a garden picket fence are quite different from those of a wire-mesh fence on a farm. The depth you must dig to properly set a fence post depends on both the post and the task of the fence.
Wood fences remain erect because of the resistance of the ground pressing against the post. A deeper post hold means more of the post is in the ground, and there is more resistance. Conversely, a fence post with a wider base does not need to be as deep as a post with a thinner base.
The standard rule for wood fences posts is to dig a hole down to a depth equal to between one-third and one-half of the length of the post. So, a hole for 1.5 metre (5 feet) picket fence posts needs to be at least 50 cm (20 inches) deep, while a hole for 2.75 metre (9 feet) fence post needs to be at least 90 cm (3 feet) deep.
Setting a wood post in a concrete plug widens its base, increasing the resistance of a post without increasing the depth. Even in fences where most of the line posts are planted in the ground, it is common to set the posts such as terminal posts and gate posts in concrete, as these must carry extra weight. Pouring concrete plugs increases the strength of the post without increasing the depth of the hole.
Sometimes it is necessary to put fence posts on to foundations, especially in low, wet areas with poor drainage. In such cases, an extra 15 cm (6 inches) inches is dug out of the post hole, which is then filled with gravel or sand. This deeper foundation is sometimes also put into place prior to pouring concrete plugs.