1. Locate the position of the posts to prepare for digging
The "opening size" of your gate is the distance between the inside of the gate posts. The gate finish size will be smaller to allow for the hinges and latch. Since the hinges and latch types may vary the space allowed for them will vary. The only measurement you can trust is what we call the "opening size". Use this measurement to space the two gate posts. Measure across your opening on the ground and mark the ground indicating the edge of each post. A 16' opening gate would have two marks 16' apart. Now to better visualize where the post must go, draw the post on the ground. A 6" sq. post would require that you draw a 6" square with the edge of the post on your opening size mark. See the sketch below.

2. Draw a circle around the gate post marks
The size of the circle, which represents the area to be excavated, should be larger than the post to allow for the proper amount of concrete. Multiply the diameter of the post at its widest point (diagonal on a square) by three and draw a circle with the post as the center using that calculation. If you are going to drill with an auger, simply mark the center of the drawn post marks to indicate the center of your auger. Now you have located the position of the holes to be dug.

3. Dig the holes
Dig the holes straight down to a depth of 36" minimum to 48" maximum. The depth required depends on the frost line depth in your area and the size of the gate. The depth of the hole is critical in avoiding leaning posts and sagging gates. Use the 48" depth for wide gates over 96" wide or heavy gates over 100 lbs. A 42" depth works for nearly any gate in most areas. If you are not sure, dig deeper. You only have to dig the holes one time--if you do it right. On small diameter holes, 6"-9", you could be limited to a depth of 36", because you can't get tools into the hole to clean it out. That is fine, if you followed the guidelines set forth thus far. The post will be shorter than the depth of the hole, i.e. 36" deep hole with 24" of the post in it. This is not unusual since the post does not need to extend the full depth of the hole. A common error is to assume the hole only needs to extend the length of the post. Make the shape of the hole so that the diameter at the bottom of the hole is wider than the top diameter. This we call "belling" since it resembles the shape of a bell. This will anchor the post and help to avoid post heaving due to frost. See below sketch. Under no circumstances should your hole resemble a carrot shape where the top of the hole is larger in diameter than the bottom.