How many bags I need to put 8 feet fence post?

Our recommendations are that the hole diameter should be 3 times the post width/diameter (for a 4x4 post, would mean at least 12 inch diameter) and the depth of the post hole should be one-half of the above ground post height (48 inches deep hole would mean only 96 inches above the ground). Any wood that is going to be set in concrete should be classified as “rot resistant wood”. Certain woods are naturally more rot resistant than other types, while some woods, sure as pine, must be treated in order to be considered rot resistant. Contact your local Home Depot store for assistance in determining what rot resistant posts are available in your area. A 60 lb bag of QUIKRETE® Fast Setting Concrete Mix yields 0.45 cubic feet when placed. A 48 inch deep by 12 inch diameter hole with a 4X4 in place will require 2.7 cubic feet of concrete, or 6 bags (of 60 lb each) of QUIKRETE® Fast Setting Concrete Mix. Yields will vary based upon waste, uneven subgrade, etc.