What Size Flies to Use for Trout

Size 14 and 16 cover most trout fishing on small water. Go to 18 and smaller in low, clear conditions, and up to 10 or 12 for terrestrials and anything you want to float a dropper. If you want to own the whole range in one purchase, the sizes 10 to 18 assortment is the cleanest way to do it; if you want to learn how much size matters, buy one pattern in four sizes and fish them against each other.

Fly size is the most under-discussed decision in fly fishing and probably the most important. An angler with the wrong pattern in the right size catches fish. An angler with the perfect pattern two sizes too large gets refusals and concludes the fish are not feeding. On small creeks the mistake runs one direction almost every time: the flies are too big, because big flies look impressive in the box and small ones look like a mistake.

The short answer

If you buy one size, buy 16. It is small enough to get eaten in clear water and big enough to see and to tie on without reading glasses. A size 16 Parachute Adams is probably the single most useful trout fly in existence — the full case for it is in the dry fly roundup.

How hook sizes actually work

The number runs backwards: bigger number means smaller fly. A size 22 is tiny — smaller than a grain of rice. A size 4 is a substantial streamer. Everything useful for trout sits between those two, and the step from one size to the next is roughly a 20 to 25 percent change in hook length, so two sizes is a visibly different fly.

Trout sizes are even numbers only. You will see 4, 6, 8, 10, 12, 14, 16, 18, 20, 22. Odd sizes exist in some hook ranges but are not part of the normal trout fly world, so a box advertising sizes 12 to 18 is giving you four sizes, not seven.

Above size 1 the numbering flips to aughts. Hooks larger than size 1 are written 1/0, 2/0, 3/0 and so on, and there the bigger number means a bigger hook again. You will never need this for creek trout; it matters only if you wander into bass or saltwater patterns. For reference, the bass fly rod guide deals with that end of the scale.

Size is not standardized between manufacturers. One company's size 14 can sit visibly next to another's size 12, because the number describes the hook the fly was tied on, not the finished fly, and shank length and gape vary by hook model. This is a real annoyance and the reason to buy a spread of sizes once and compare them in your own box rather than trusting labels across brands.

Sizes by fly type

Nymphs run one size smaller than people expect. The real insects living in a creek bed are mostly small — a size 16 or 18 Pheasant Tail is a realistic mayfly nymph, while a size 10 is a stonefly. If your nymph box is all 12s and 14s, adding 16s and 18s will catch you more fish than adding more patterns will. The nymph roundup covers which patterns to buy in those sizes.

Streamers are a separate decision, not a bigger fly. Sizes 4 to 10 are not there because the trout are bigger; they are there because a streamer imitates a fish or a crayfish rather than an insect, and it is fished with movement instead of drift. On a narrow creek a size 8 or 10 is usually the practical top end — the streamer roundup goes into which ones work in tight water.

What the water tells you

Clear and low: go smaller. Clear water gives a trout time and light to inspect the fly, and the easiest thing to get wrong at that range is bulk. In the low flows of late summer, dropping from a 14 to a 16 or 18 converts refusals into takes more reliably than any pattern change — the summer fly roundup builds a whole box around that constraint.

High and stained: go bigger. After rain the fish cannot see far, so the fly has to be findable. A size 10 bead head, a size 12 worm pattern or a size 8 Woolly Bugger moves enough water to be noticed. This is the one situation where a big fly genuinely out-fishes a small one on a creek.

Cold water: go smaller again. In winter and early spring the insects that are active are mostly midges, which is why size 18 to 22 patterns dominate cold-weather fishing. Trout metabolism is slow, they will not chase, and a tiny fly drifted right past them is the only realistic offer.

Wind and broken water: go bigger, within reason. You cannot fish what you cannot see, and a size 20 midge on riffled water in a breeze is invisible to you even if the fish would eat it. Size up to something you can track, or fish the small fly as a dropper under a visible dry so the dry does the watching for you.

Matching tippet to fly size

Divide the hook size by four to get the tippet X. A size 16 fly wants about 4X, a size 20 wants 5X, and a size 12 is happy on 3X. It is a rule of thumb rather than physics, but it keeps you out of the two failure modes: tippet so thick the small fly will not drift naturally, and tippet so fine that a decent fish breaks you off.

The symptom of a mismatch is a fly that will not sit right. A size 20 midge on 3X tippet hangs stiffly and drifts unnaturally, because the leader is too heavy to let the fly move with the current. Going the other way, a size 8 streamer on 6X tears off on the first good fish. If the X system is new to you, it is decoded in fly line, leader and tippet explained, and the knot to join them is in how to tie tippet to leader.

Change size before you change pattern

This is the single most useful habit on this page. When a trout rises to your fly and does not take it, the fish has told you it is feeding and has looked at your offering closely enough to decline. Nine times in ten the first correction worth making is the same fly one or two sizes smaller, not a different insect. Pattern changes are the second move, not the first.

Why smaller works. Size is the easiest thing for a trout to judge and the hardest for a fly to disguise. Color and silhouette are seen through a distorted surface and in motion; relative size is read instantly against the real insects drifting alongside yours. A fly that is too large reads as wrong even when the pattern is right.

Carry fewer patterns in more sizes. Three patterns in four sizes each is a far more useful box than twelve patterns in one size, and it costs the same. That is the argument for buying single-pattern multi-size sets like the four-size Stimulator pack or the Pheasant Tail ladder rather than another broad assortment once you own one.

And write down what worked. Size, pattern, time of day, water height. Four entries in a phone note across a season will teach you more about your own creek than any chart can, because fly size is ultimately local: the insects in your water have their own sizes, and those are the ones that matter.

What to buy first

One mixed box spanning 10 to 18, then a midge set when you hit the wall. The sizes 10 to 18 assortment covers every normal creek situation, and when you meet the fish that refuse all of it — a flat clear pool on a bright afternoon — the sizes 18 to 22 midge box is the next purchase rather than a wider pattern selection.

Buy doubles of your workhorse sizes. You will lose 16s in trees and leave 18s in fish. Nothing is more frustrating than finding the size that works and having one of it. The cheapest way to stock up on the sizes you actually fish is covered in the cheap flies roundup.

Then sort the box by size, not by pattern. Rows of ascending size make the decision on the water a physical one — you reach one row left to go smaller. Fly boxes with slit foam and clear lids make that practical, and the fly box roundup covers which ones hold small flies without losing them. If you are assembling a first kit from scratch, start with the beginner fly roundup instead and come back to sizing once you have fished a few trips.