The depth of your post for “in ground” applications ultimately is based upon your local building codes. We recommend, at minimum, that your posts should be at least 2 feet longer than the height of your fence or gate; however, if you’re in an area of the country with frost line issues, you may be required to order longer posts. It’s very easy to check with your HOA or your local building codes to determine what’s required in your, specific local. And, we recommend doing so.
The amount of cement required for each hole will depend upon a) how deep your posts will be sunk and b) how wide a hole you dig for each post. Most customers use a 6″ or 8″ auger for their posts but you’ll have to make adjustments on-site based upon the two variables listed above. There is no set rule for this and it’s different based upon the application, location, size of post, and local building codes.