How does hook size relate to insect length?

Fly-tying hooks use even-numbered sizes where a larger number means a smaller hook. A standard dry-fly hook's shank length in millimetres roughly equals the insect body it imitates, so a size 14 (about 10 to 12 mm shank) suits a 10 to 12 mm mayfly.

Why are hook sizes counterintuitive?

The numbering descends as hooks shrink: a size 8 is large, a size 22 is tiny. The system is historical and based on gape width. The tool converts insect length directly to the right number so you do not have to memorise the scale.

Should I match exactly or go smaller?

When fish are selective during a heavy hatch, match the size precisely or go one size smaller, as trout often refuse oversized imitations. A fly that is slightly too small is usually accepted while one too large is refused.

Do all hook brands size the same?

Roughly, but not exactly. Wide-gape and curved scud hooks can fish a size differently from a standard dry-fly hook, and brands like Tiemco, Daiichi, and Mustad vary slightly. Use the size as a starting point and compare hooks side by side.

What about terrestrials and large nymphs?

Large terrestrials such as grasshoppers and big stonefly nymphs run 20 to 40 mm and use sizes 4 to 10, often on long-shank or hopper hooks. The tool covers the full common range from tiny midges at size 24 up to streamers at size 2.

Fly Tying Hook Size & Hatch Match Reference

Matching the hatch starts with putting your fly on the right size hook. This reference converts a measured natural-insect body length into the standard fly-tying hook size and lists the common hatches that fall in that size range, so you can tie a believable imitation.

How it works

Fly-tying hooks are numbered on a descending even scale — a size 8 is large and a size 24 is tiny — based historically on gape width. For a standard dry-fly hook the shank length in millimetres tracks the insect body it imitates closely enough that matching is direct:

larger number  → smaller hook
hook shank (mm) ≈ insect body length (mm)

The tool maps your entered body length onto that scale and returns the best-fit size (plus the neighbouring size for going one smaller when fish are selective). It then filters a table of common hatches to those whose adults sit in the same length band.

Tips and notes

When trout are keyed in during a heavy hatch, match the size precisely or drop one size smaller — fish refuse oversized flies far more often than slightly small ones. Remember that brands and hook styles vary: a wide-gape scud or curved nymph hook can fish a size differently from a standard dry-fly hook, so treat the recommendation as a starting point and compare hooks side by side. Measure the body only, ignoring tails and antennae, since those add length without changing the fly profile fish key on.