As explained on the previous page, it is the length of the hook shank which decides the size of the fly. The shank/gape ratio (the shank length divided by the gape) is a clear indicator of this aspect of a hook but for easier reading I have defined arbitrary ranges as follows:
Examples of the consequence of this being not clearly defined:
(referring to the table on the page 'Hook comparison')
The next to last item, Varivas 2300 Ultra Midge is stated to be Size 26 and yet the gape defines it as actually Size 20.
It certainly gives a very small fly, since the Shank/Gape ratio is 0.8, the shortest shank on the list.
The manufacturers are clearly confused as to how to size the hook. In fact its gape is exactly the same as the Kamasan B410 Size 20 which is correctly sized.
The last item on the list, also Kamasan B410, is correctly sized as Size 22, and yet has a smaller gape than the so-called Varivas Size 26. Its Shank/Gape ratio, however, is 1.2 Medium which will give a 'size 22 fly'.
The following graph shows suggested limits for hook size (horizontal scale numbered along the bottom) relative to the gape measured in mm (vertical scale). This is based on the average of the majority of hooks which are close to or within these limits.
I use this graph to classify hooks for direct comparison.
The next graph shows the division of hook weights according to the measured wire gauge.
Wire gauge is, of course, related to hook size so that e.g. size 10 in 0.6mm wire is Medium but size 12 in 0.6mm is Heavy.
Note that the smaller a hook is, the less important becomes the wire weight since a small hook is bound to be relatively lightweight.