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.
Hook size and insect length reference
| Insect body length | Standard dry-fly hook size | Typical hatch examples |
|---|---|---|
| 3–4 mm | Size 24–22 | Tiny winter/spring midges, BWO spinners |
| 5–6 mm | Size 20–18 | Trico, small Baetis (Blue-Winged Olive) |
| 8–10 mm | Size 16–14 | PMD (Pale Morning Dun), Hendrickson, Light Cahill |
| 11–14 mm | Size 12–10 | March Brown, Green Drake emerger |
| 15–20 mm | Size 8–6 | Large stonefly adults, big caddis |
| 25–40 mm | Size 4–2 | Giant stonefly (Pteronarcys), hopper patterns |
These are body-only lengths — tails and antennae are not included.
When to go one size smaller
Trout during a heavy hatch become highly selective and key on the size and silhouette of the natural. A common rule among experienced dry-fly anglers: match the natural size precisely on the first cast, then if fish refuse, drop one size smaller before changing pattern. Oversized flies are almost always refused; undersized ones are usually accepted. This is especially true for Trico and midge hatches, where a size-too-large fly is immediately obvious to fish feeding in still or slow water.
Hook style matters alongside size
The shank length (which governs apparent fly size) varies by hook style, so two hooks of the same number from different styles may fish differently:
- Standard dry-fly hooks (Tiemco 100, Daiichi 1180) — the reference style for most size-to-insect matching.
- Wide-gape dry-fly hooks — a shorter shank for the gape size; ties a slightly smaller-looking fly than the size would suggest.
- Long-shank hooks (e.g., 2XL nymph hooks) — same size number but the shank is longer, making the fly body look bigger. Used for nymphs and streamers where extra body length helps.
- Curved scud hooks — the bend curves the shank, making the natural fly length shorter than a straight hook of the same size number.
- Short-shank midge hooks — specifically designed for very small patterns, offering a wider gape relative to shank length for better hooking on tiny flies.
Measuring a natural accurately
Catch or net an insect and place it alongside a ruler or the mm scale on your hook holder. Measure from the front of the thorax (not the eye) to the end of the abdomen, ignoring any tails. Wings folded back over the body add apparent bulk but not hooking length, and fish key on the body outline. For consistency, measure several specimens if possible — individuals of the same species vary in size through a hatch.
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.