Reference
Fly hook sizes explained
The numbering is counter-intuitive, inconsistent between makers, and worth ten minutes of your life.
Bigger number, smaller hook
Fly hooks are numbered so that the size goes down as the number goes up. A #10 is a substantial hook; a #22 is smaller than a grain of rice. The scale runs in even numbers for trout sizes — 10, 12, 14, 16, 18, 20, 22 — and each step is genuinely noticeable on the water.
Then, above #1, it flips. Sizes continue as 1/0, 2/0, 3/0 (said 'one aught'), and now the number going up means the hook is getting bigger again. Saltwater and pike flies live here. Nothing about this is logical; it is simply the convention.
What to carry for trout
| Size | Use | Examples |
|---|---|---|
| #6–#10 | Streamers, big nymphs, hoppers | Woolly Bugger, Clouser, Stonefly |
| #12–#14 | Everyday dries and nymphs, the workhorse range | Parachute Adams, Prince Nymph |
| #16–#18 | Smaller hatches, clear water, wary fish | Pheasant Tail, small Adams |
| #20–#22 | Midges, tailwaters, spring creeks | Zebra Midge |
Sizes are not standard between manufacturers
A #14 from one hook maker can be visibly larger than a #14 from another, because the number describes a maker's own scale rather than an agreed measurement. This is why serious fly buyers care which hook a fly is tied on, and why naming the hook is worth real money on a listing.
It is also why you should judge a fly against the naturals rather than against the number on the box.
The two-size rule
When a fish refuses a fly it clearly saw, drop two sizes before you change anything else. It fixes more refusals than changing pattern, changing colour, or changing position — and it costs you one fly change.