Steel posts come in many shapes and sizes. The most common post for wood fences in the Dallas area is 2 3/8 inch diameter galvanized. There are also many thicknesses of steel that can be used to make the post. The two most common thicknesses are 0.065 (15 Gauge) and 0.095 (CS-20); the greater the number the thicker the steel.
The distance between the posts, the location of wind breaks, and the height of the fence…each play a role in determining the thickness of a post. While a 6 foot fence located against an alley and lined with shrubs might be fine with 0.065, and the 8 foot fence off to one side of a yard does well with posts 6 feet apart and 0.095 thick, the 8 foot fence along an open street might need posts to be spaced 5 feet apart with a steel thickness of .120 (schedule 40 Standard pipe). In extreme load conditions, the diameter of the pipe could be increased, thus causing a exponential increase in the rigidity of the fence post.