What dimensions should the hole be for the posts?

For the mailbox post, you need to dig a hole 30″ deep, and minimum 12″ diameter. Keep in mind, if you have curbing that is above the street level, you will need to add this height to the depth of the hole. Please check with your Post Office to find what their regulations are on mailbox height, and proximity to the road and adjust accordingly. For the lamp post, you will need to dig a hole 30″ deep, and minimum 12″ diameter. Before starting any digging projects, state law requires that you contact Dig Safe, 1-888-344-7233 (888-dig-safe).