The unfortunate reality is that hook sizes are not standardized.
Hook Comparison Chart
There are several problems. The size of the hook is determined by the gape, the distance from the point of the shank. However, what we use to match an insect is the length of the insects body which correlates with the hook shank length and not the hook gape. That is a major problem.
The second problem is that each manufacturer sets it's own size and shape. So the gape and associated shank lengths can vary, especially in the smaller hook sizes, between various manufacturers. The most common hook scale is the Reddich hook scale but not everyone follows it.Even the Reddich scale varies from old to new.
Fly Fishing History: Hook Scales
The final problem is the hook shape and where one begins to tie the body of the fly. Body lengths for identical hook sizes can vary depending on which hook shape is used.
"Modern systems concentrate on standardising the length of the shank, but this is a difficult measurement to make of a hog-backed hook, and differences in bend and eye diameter may conspire to make a small hook appear larger than its official size. No system can take account of hooks which are classified differently by custom, for example long shank trout hooks, which take their size from the gape of the hook, rather than the length of the shank. Consider the unofficial extension of the current system of measurement to account for sizes below size 16, and we are only marginally in advance of the chaos of the nineteenth century. As long as different makers continue to produce different patterns of hook, we are unlikely to see any improvement."
Fly Fishing History: Hooks
Fly size
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