How many bags will i need for 16 fence posts?

A 60 lb bag of QUIKRETE® Fast Setting Concrete Mix yields 0.45 cubic feet when placed. The number of bags required for setting 16 fence posts will depend on the post hole diameter, post hole depth, and size of the posts. Our recommendations are that the hole diameter should be 3 times the post diameter/width and the depth of the post hole should be one-half of the above ground post height. For example, sixteen 4X4 fence posts that will be 6 feet above ground placed into 12 inch (1 foot) diameter post holes that are 3 feet deep will require 32.4 cubic feet of concrete, or 72 bags (of 60 lb each) of QUIKRETE® Fast Setting Concrete Mix. Yields will vary based upon waste, uneven subgrade, etc.