| If this is a homework question, there's not enough info to answer it. Now, I don't know what the formula would be, but no doubt you'd need at least the shape and size of the posts as well as the shape size and weight of the fence itself, whose wind resitance would also affect the posts.
If, on the other hand, it is a question about a chain link fence you are putting in, then please clarify: Do you have existing concrete already there where the posts would go to bolt into? | 
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