Gate posts are often set into concrete foundations for extra stability, as the post must bear the weight of the gate as well as serve as a terminal post in the fence line. The concrete post foundation, sometimes called a "plug," widens the base of the post and giving it more resistance against the ground. Pouring a concrete gate post foundation entails only a few extra steps beyond what is necessary for setting a fence post in the ground, making it only slightly more time-consuming.
Dig the post hole with a shovel, post hole digger and a mattock. The width should be triple the width of the gate post, and the depth between 1/3 and 1/2 the height of the post, plus 15 cm (6 inches). So, if you have an 20 cm (8 inch) wide, 2.4 m (8 foot) high post, the hole should be 60 cm (24 inches) wide and at least 95 cm (38 inches) deep.
Fill the bottom 15 cm (6 inches) of the post hole with either sand or gravel. This will serve as the foundation for the concrete plug.
Mix the quick-setting concrete in a bucket, wheelbarrow or whatever vessel is convenient.
Set the post into the post hole. Check it with a level to ensure it is standing straight up.
Pour the concrete into the post hole, filling it to about 7.5 to 10 cm (3 to 4 inches) from the top. Check the gate post's straightness with the level again, adjust as necessary, and hold the post in place for 15 to 20 minutes.
Stop supporting the post once the concrete has dried enough to support it. Allow the concrete to harden overnight, and then fill in the remainder of the post hole with dirt.
Instead of holding the gate post in place by hand while the concrete dries, you nail two supporting legs to the bottom of the post, forming a triangular support base. However, if you only need to set two gate posts, you will find cutting and arranging these support legs takes as much time as holding the post up by hand.