This is a Table of the various ways to measure leader size, including the French system.....

 

Size in X Size in Inch (.0011-Size in X *.001) Size inMeters Centieme mm
1 0.010 0.000254 25.4 0.253807
2 0.009 0.000228 22.8 0.228426
3 0.008 0.000203 20.3 0.203046
4 0.007 0.000178 17.8 0.177665
5 0.006 0.000152 15.2 0.152284
6 0.005 0.000127 12.7 0.126904
7 0.004 0.000102 10.2 0.101523
8 0.003 0.000076 7.6 0.076142