OD (outside diameter)
OD or outside diameter is the diameter of round steel pipe or tubing that is used in the construction of various kinds of fence.
OD can be expressed in metric or English dimensions, and the bigger the diameter the bigger the pipe or tubing is.
If you are not sure what the diameter of your pipe or tubing is, you can use a caliper or vernier as pictured, or you can measure the circumference of the pipe and use it to calculate the diameter.