Choosing a fly size, and what the numbers mean
Explainer · Last reviewed 2026-08-14
Fly size is hook size, the scale runs backwards, and it is not standardised between patterns or manufacturers. Size matters more than colour and is chosen from what the fish are eating.
Fly size is the property anglers change least and that most often decides the outcome.
THE NUMBER IS A HOOK SIZE AND THE SCALE RUNS BACKWARDS. A size 18 fly is smaller than a size 12, which is smaller than a size 6. Below size 1 the scale reverses again into the aught sizes — 1/0, 2/0, 3/0 — which get progressively larger, and those are the sizes used for pike, saltwater and salmon patterns.
THE SCALE IS NOT STANDARDISED. A size 12 in one manufacturer's dry-fly pattern and a size 12 in another's heavy nymph hook can differ noticeably in gape, shank length and wire weight. The number is a strong guide within a range and a weak one across ranges, and there is no authority that harmonises them.
SHANK LENGTH IS RECORDED SEPARATELY where it matters, as a modifier: a long-shank hook of a given size carries a longer dressing on the same gape. That is how a streamer is tied large without a hook gape that damages the fish's mouth.
SIZE IS CHOSEN FROM THE FOOD, NOT FROM THE FISH. A large trout eating tiny midge pupae wants a tiny fly. The commonest single error in fly fishing is fishing a fly too large because the fish is large, and the second commonest is failing to go smaller when a fish refuses.
AND SIZE MATTERS MORE THAN COLOUR, WHICH IS WORTH SAYING PLAINLY. Silhouette and size are what a fish resolves first, particularly in poor light, coloured water or at depth, where colour discrimination falls away before shape does. Practitioner conviction about exact colour shades is strong and the evidence supporting it is much weaker than the evidence for size and profile.
IN SALT WATER THE SIZES ARE LARGER AND THE REASONING IS THE SAME. A sandeel pattern is matched to the sandeels present, and 'match the bait' fails in exactly the way 'match the hatch' does when the imitation is bigger than the real thing. A fly noticeably larger than what the fish are eating is a common reason for follows without takes.
The numbers run backwards, and size matters more than pattern
Hook sizing confuses everybody at least once and the confusion is structural rather than personal.
THE NUMBERS RUN BACKWARDS. A size 20 fly is tiny and a size 6 is large — the number goes UP as the fly goes DOWN, until the scale crosses into the aught sizes for genuinely large hooks, at which point 1/0, 2/0, 3/0 go up as they get bigger. There is no logic to recover here; it is a historical convention and it has to be learned rather than derived.
AND IT IS NOT STANDARDISED BETWEEN MAKERS. A size 12 in one manufacturer's pattern is not a size 12 in another's, because the number indexes a hook within a pattern range rather than a measured dimension. This catalogue records that as the reason a hook size guide remains a blocked tool: the only datasets that exist are individual manufacturers' charts.
WHAT ACTUALLY MATTERS IS THAT SIZE BEATS PATTERN. Where fish are feeding on something specific, matching its SIZE is the single most important variable, ahead of silhouette, ahead of colour and far ahead of pattern name. An ordinary pattern in the right size out-fishes a perfect one in the wrong size consistently enough to be a rule.
AND SIZE INTERACTS WITH CASTING. A big wind-resistant fly needs a heavier line, a steeper leader and a tighter loop; a tiny fly needs a fine tippet and a delicate turnover. Choosing a fly is therefore also choosing what the rest of the outfit has to do, which is why an angler carrying one outfit is limited in what they can present.
THE PRACTICAL RULE ON A COAST OR A STILLWATER: start with the size of what is actually in the water — look at the fry, look at the shrimps — and change size before changing pattern. On a river during a hatch: catch one of the insects and match its length rather than its species.
AND A NOTE ON HOOK WEIGHT. The same nominal size in heavy wire sinks and in fine wire hangs; on a dry fly that difference decides whether it floats at all. Wire gauge is part of the size decision and is rarely printed anywhere obvious.
Practitioner consensus
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
- Still Water Fly-Fishing — Andre Deutsch, 1952 (Specialist angling publication)Relied on for: Fly-fishing practice from a practitioner tradition: line control, presentation, depth management and the reading of water.States this directlyCitation checked — the work exists and is described correctly
- Trout & Salmon — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK game angling practice as reported by practitioners: tackle, casting, presentation and river craft.States this directlyCitation checked — the work exists and is described correctly
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated descriptions of technique and tackle convention, recorded as consensus rather than as scholarship.Background and mechanism onlyCitation checked — the work exists and is described correctly
Where to go next
Fish this applies to
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Grayling
A fish or a water joined to the fly-fishing mechanism it is a worked example of, written only where the subject's own behaviour or physical character is what the record explains. The direction is the one the predicate table allows and the one the graph already uses 176 times, and it is also the semantically correct one: the mechanism explains the fish, not the reverse.
- Still Water Fly-Fishing — Andre Deutsch, 1952 (Specialist angling publication)Relied on for: Stillwater fly practice: line density and depth, leader construction, fly selection against the food present, and the behaviour of stocked and wild trout in still water.States this directlyCitation checked — the work exists and is described correctly
- Trout & Salmon — Bauer Media (Specialist angling publication)Relied on for: Long-running published description of British game angling practice: river and stillwater trout, grayling and salmon fishing, and the presentation each demands.States this directlyCitation checked — the work exists and is described correctly
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Life history, feeding biology, habitat requirement and seasonal behaviour of the freshwater fishes of the British Isles.States this directlyCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Recorded environment, depth range, feeding biology and distribution of the species named.States this directlyPassage read and verified
Where it matters
- Fly fishingFly size is chosen from the food rather than from the fish, and it is the property anglers change least and that most often decides the outcome.
- NymphingWhere size is most consequential and least intuitive, because the fly is not visible while fishing. A nymph is chosen by weight and size together, and the two are frequently traded against each other to reach a depth.
What these connections rest on (2)
Practitioner consensus — 2 connections
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Fly fishing
A mechanism edge between a record and the explanation beneath it. Fly tackle and technique rest on established practitioner craft rather than on experiment, and the support level says so rather than borrowing an authority the subject does not have.
- Trout & Salmon — Bauer Media (Specialist angling publication)Relied on for: Contemporary game and fly angling practice as reported and restated by practitioners, including tackle convention and technique.Background and mechanism onlyCitation checked — the work exists and is described correctly
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated angling practice and belief, recorded as practitioner consensus rather than as tested fact.Background and mechanism onlyCitation checked — the work exists and is described correctly
Nymphing
A fly technique joined to the mechanism it depends on. Ten fly concepts carried no edge to any method, which left the vertical's explainers and its techniques as two halves that could not be navigated between.
- Still Water Fly-Fishing — Andre Deutsch, 1952 (Specialist angling publication)Relied on for: Stillwater fly practice: line density and depth, leader construction, fly selection against the food present, and the behaviour of stocked and wild trout in still water.States this directlyCitation checked — the work exists and is described correctly
- Trout & Salmon — Bauer Media (Specialist angling publication)Relied on for: Long-running published description of British game angling practice: river and stillwater trout, grayling and salmon fishing, and the presentation each demands.States this directlyCitation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: British sea angling practice including shore fishing for bass, pollack and mullet, the ground each is found over and the presentation each takes.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
- FAO Species Identification Guides for Fishery Purposes and the FAO Species Catalogues — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Distribution, habitat and feeding biology of coastal and tropical marine species, from regional identification guides prepared for fishery purposes.States this directlyCitation checked — the work exists and is described correctly
How this compares
- Matching the hatch: what it means and when it is wrongSize is the part of matching the hatch that reliably works, and it is the part most often abandoned in favour of colour.
- The four fly types, and what each imitatesType and size are the two properties of a fly that matter before anything else, and colour is a distant third.
- Tippet size, and when to go finerTwo backwards-running numbering conventions inherited from manufacturing rather than from fishing, and both are routinely misread.
What these connections rest on (3)
Practitioner consensus — 3 connections
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Matching the hatch: what it means and when it is wrong
Two records a reader would reasonably weigh against one another, joined where the comparison teaches something about both. A navigation claim rather than a claim about the world.
- Trout & Salmon — Bauer Media (Specialist angling publication)Relied on for: Contemporary game and fly angling practice as reported and restated by practitioners, including tackle convention and technique.Background and mechanism onlyCitation checked — the work exists and is described correctly
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated angling practice and belief, recorded as practitioner consensus rather than as tested fact.Background and mechanism onlyCitation checked — the work exists and is described correctly
The four fly types, and what each imitates
Two records a reader would reasonably weigh against one another, joined where the comparison teaches something about both. A navigation claim rather than a claim about the world.
- Trout & Salmon — Bauer Media (Specialist angling publication)Relied on for: Contemporary game and fly angling practice as reported and restated by practitioners, including tackle convention and technique.Background and mechanism onlyCitation checked — the work exists and is described correctly
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated angling practice and belief, recorded as practitioner consensus rather than as tested fact.Background and mechanism onlyCitation checked — the work exists and is described correctly
Tippet size, and when to go finer
Two records a reader would reasonably weigh against one another, joined where the comparison teaches something about both. A navigation claim rather than a claim about the world.
- Trout & Salmon — Bauer Media (Specialist angling publication)Relied on for: Contemporary game and fly angling practice as reported and restated by practitioners, including tackle convention and technique.Background and mechanism onlyCitation checked — the work exists and is described correctly
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated angling practice and belief, recorded as practitioner consensus rather than as tested fact.Background and mechanism onlyCitation checked — the work exists and is described correctly
Sources for this page
- Freshwater Fishing — Macdonald & Jane's, 1975 (Specialist angling publication)Relied on for: Comprehensive freshwater angling practice including fly fishing tackle, casting, presentation and the reasoning behind terminal tackle choices.States this directlyCitation checked — the work exists and is described correctly
- The Ecology of Vision — Clarendon Press, Oxford, 1979 (Scientific literature)Relied on for: The ecology of vision in aquatic animals, including how light level, water colour and turbidity affect what a fish can see and at what range.States this directlyCitation checked — the work exists and is described correctly
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated angling practice and belief, recorded as practitioner consensus rather than as tested fact.Background and mechanism onlyCitation checked — the work exists and is described correctly
How FishingHQ labels evidence is explained in the evidence policy.