It is recommended that you bury the post two to three feet into the ground. Yet, when you go to the field, most sign posts are actually only buried 18" into the ground.
How deep should a sign post be?
The depth depends on the length of the post. For example, a 4-foot sign post should be set 1.5 feet deep, while an 8 to 12-foot post should be driven 3.5 into the ground. Keep in mind that a damaged or broken sign post is easier to remove if it is not set into the ground more than 3.5 feet.