I once had a link to a site that explained how the Japanese line size system was originally developed, but the history of it doesn’t tell you what you need to know now.

For all practical purposes, you can multiply the Japanese line size by 4 and get the equivalent pound test for US fluorocarbon spinning line, so size 4 line is essentially 16# test, size 3.5 is roughly 14# test and so on. That holds until you get down to tippet material, where advances in technology make the smallest sizes stronger than that quick ratio would suggest.

The tippet size equivalents are as follows:
X number – Japanese line size
1X – 2.5
2X – 2
3X – 1.5
4X – 1
5X – .8
6X – .6
7X – .4