How to Select the Right Fly Size

Fly fishing is an ever evolving art and one that requires constant learning and adaptation. To paraphrase a good friend and seasoned guide, if you think you know everything there is to know about fly fishing you’re insane and you’ve already lost. His words weren’t quite so PC, but you get the point. As anglers, we never stop learning and honestly, we love that about the sport! At FlyCast, it is our mission to help anglers of all skill sets have the most productive day on the water and to provide as much information as humanly possible to help bridge that gap.



With a number of new hatches materializing this summer, we’ve been thinking a lot about bugs and the flies that we use to imitate them. While choosing the right pattern is imperative to success, the size of said fly is equally important. In this blog, we’ll discuss how to select the right fly sizes based on the various hatches in an effort to increase your odds of landing more trout.


Look for yourself: The most effective way to determine the appropriate fly size is to simply look for yourself. Given little to no understanding of aquatic bug life, you can, with relative accuracy, determine the right fly and size by analyzing what is on the water as it relates to what’s in your fly box. In some cases, this is easier said than done, but if you can manage to capture surface flies, turn over a few rocks or use a bug seine, you can do a side by side comparison and get reasonably close.



Do your homework: Understanding the various bugs in the water along with their relative size range will help narrow it down and drastically improve your odds before you even hit the water. While bug size varies by location and season, the following breakdown provides a good rule of thumb when determining the right flies for the various Colorado rivers and streams.


Midges: Size #20 - 26

  • Midges are present year round, but do vary in size depending on the season. In the warmer months (June through August), comparatively warmer water temps lead to bigger bugs. As such, choose flies in the #18 to #22 range. In the winter/spring, however, you’re going to want to think small and choose flies in the #24 to #26 range, especially if you are fishing a tailwater. In general, smaller is typically better when choosing the appropriate midge pattern.


Baetis: #18 - 22

  • Baetis typically hatch between April and October, but become increasingly present through the summer and fall. In general, you’re looking for patterns in the #20 to #22 range. However, some baetis, like a green drake or pale morning dun, will often grow bigger, particularly in warmer water, and can be found in a size #16 to #18

Caddis: #14 - 18

  • While there are a variety of caddis hatches throughout the year, they are most prevalent between May and August. Early in the season, the mother's day caddis emerge. This is an incredible time of year and while the window is short, it produces some great dry fly fishing. During this time, you’ll want to go with caddis in the #14-16 range, but we tend to side on the smaller side. Beyond that, sizes range from #14-18 among the sedge and micro caddis variations.

Stonefly: #2 - # 12

  • Again, there are a variety of stoneflies present throughout the year and sizes vary greatly. Typically, you’ll find stoneflies present between late May and early September. Stoneflies are unique in that they often live as nymphs for one to three years crawling on the riverbed. When they are mature enough to hatch, these insects will make their way to the edge of the water along the banks, going so far as nearby trees or shrubs, and split their nymphal husks and become adults. Early season stoneflies, like salmonflies or golden stoneflies, tend to fall in the #8 to #12 range, but can get as big as a #4. Otherwise, later in the season, yellow sallies tend to be somewhat bigger and fall in the #2 to #8 range.


Big Fish Big Fly: While the aforementioned guidelines provide a general overview for choosing the right fly size based on the time of year and hatch, fish don’t always play by the rules. Meaning, trout will often be attracted to patterns that are out of season (not hatching) or don’t necessarily imitate any particular fly. Sometimes, trout see a big meal and will act on it. So if you’ve tried your luck imitating the various hatches and fly sizes that are currently present, only to find yourself coming up short, don’t hesitate to rig up some beefy flies. This motto is particularly effective when it comes to streamers. Streamers imitate bait fish and in order to attract bigger trout, you’ll need bigger flies to make it worth their while.


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