You will often see the size of a fly represented in hook size or in mm’s. I have never easily adjusted to metric and have a rough sense of flies in the 5mm to 60mm range. But, I need to actually see the graph to be sure. I enclose these charts for visuals, realizing you will refer to a ruler, etc.
Bead Chart to Left, courtesy of Wingaersheek Flies
Natural Insect: Black Winter Stonefly (Family: Capniidae)
Imitation: Black Elkhair Caddis
Hook Size: 18-22
So, notice a 5mm fly is about the equivalent of a size 18 to 22 hook. Many references world wide will always utilize mm references for insect size, pattern size, bead size or compare hook size (standard references) to an insect size (in mm’s)