We're putting in a 6 ft. stockade privacy fence in the back yard and I'm trying to figure out what length of post we need - I've searched everywhere but no one mentions length, only witdth. We know to use 4x4, but how deep do the posts need to go into the ground (using concrete)?

If the fence is six feet high, and the top of the post sits just below the top of the fence, how deep into th ground does the post need to go? Do we need 8ft or 10ft length posts?