A mailbox post needs to be at least 2 feet deep when installed in concrete.
When installed in the ground without concrete the mailbox post often needs to be installed at least 3 feet deep.
Although you also have to factor in the height of the mailbox and the mailbox has to be high enough for the postal employee to easily reach in and deliver the mail.
2 feet or 24 inches is a common depth for mailbox posts.
The USPS also recommends that the height of mailbox be positioned 41 inches to 45 inches from the road surface to the bottom of the mailbox.
Burying the mailbox post 24 inches deep which is 2 feet deep helps to achieve the height and ensure it's also stable.
The soil conditions and the type of mailbox can also influence the depth of the mailbox post.
Heavier mailboxes may require deeper depths for installation.
The mailbox should also be placed 6 inches to 8 inches back from the curb.
Also be sure to put your house or apartment number on the mailbox as well.