flyfish dictionary
flyfish dictionary
      The Flyfish Dictionary
X
Used as a measurement to determine the thickness of a leader or tippet material. Typically used like 4X or 5X. If you want to know the exact diameter you should subtract the "X" number, so for example 4X from the number eleven.
The remaining number ( in this case 7 ) is the diameter in thousands of an inch or .007"
In the metric system .007" would be 0.1777mm so roughly 0.18mm.
Also see "rule of eleven"