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Choosing the Correct Fly Size

One question that I often get asked is how you go about choosing the correct fly size. It’s a great question that’s not the easiest to answer. The short answer is that it depends. The long answer is that what it depends on are a multitude of different things.

For this article I’m focusing solely on choosing the correct fly size in regards to fly fishing for trout. Other species may follow similar rules/suggestions that I’ll mention, but there are also different circumstances to take into consideration. Just a little disclaimer there before we get going.

Considerations When Choosing the Correct Fly Size

As you are learning how to fly fish, you’ll certainly encounter that moment when you are are standing on the banks of your favorite river or lake and your mind draws a blank as to what exactly you should tie onto the end of your leader. Right after you decide that, you then draw a blank on just what size that fly should be.

To help you along in this process, there are four main things I feel to consider when choosing the correct fly size. Those are what time of year it is, what types of insects are on, or under, the water, what your main goal is that day when out on the water, and what the fish are doing.Fly Fishing Fly Assortment - Choosing the Correct Fly Size

Choosing the Correct Fly Size For the Time of Year

The time of year can greatly influence fly size. For example, Winter months don’t have the most prolific insect hatches and the majority of insects available are midges and other small insects. That being the case, smaller size flies (18 and under) are more commonly used. On the other side, when Summer is upon us and there are many different kinds of insects, including terrestrial (grasshoppers, beetles, etc.), your fly size will generally increase.

A good rule to remember is that Winter months generally force you to fish smaller flies while the Spring and Summer months allow you to fish with larger flies

Choosing the Correct Fly Size For the Types of Insects

You may notice that the type of insects available on the waters you fish are a major player in fly size. That coincides with the seasons as previously mentioned. What you’ll want to do is start looking at those insects you see and hear about on your waters. They will generally fit into a certain size range and you can adjust your flies you use accordingly.

The prevalence of these insects does depend on your geographic location, but for the most part wherever you are at there will be some sort of insect available to help you out. If you can find a nymph under a rock or a mayfly flying around you, you can easily see an approximate size of what the fish may be looking for.

Ugly Bug Black Rubber Legs - High Water Fly Fishing FliesA quick example is if you are fishing in the late Spring/early Summer months when the stoneflies begin to emerge. If you haven’t seen one of these, they are big! You may not see any flying around you but you kick over a couple rocks and there they are. These insects generally are an inch or two long and are hard to miss. With that information, you search through your fly box for your stoneflies and find one in the size closest to what you see. This fly size will also help you determine the proper leader/tippet size as well. Check out my previous article on fly fishing leader and tippet for more information on matching fly sizes to tippets.

This same process can be applied with smaller nymphs and insects that are flying around in the air. If you see a mayfly crawling on your glasses, check the size and color and go from there.

Choosing the Correct Fly Size For Your Goals

You may be wondering what I mean by your goals for the day. Often times I will head out to my local river with a specific type of fishing I want to do. This could be to practice my European nymphing techniques, to chuck some big streamers for larger fish, or any other type of situation. Those are the goals for the day that I’m talking about and they can greatly affect what size of flies you’ll be fishing with.

Streamer Fishing Tactics

Photo by Curtis Fry

For example, you have the option of using larger “searching” flies that don’t really match up with the insects on the water. These can tend to be larger size flies used with the intention of almost making the fish strike it so that you know where they are. Sometimes I use larger nymph patterns in the 10-12 size range for this purpose. Are there insects that these patterns match in the water? Maybe, but that isn’t what I’m trying to do with them. They are mostly to get attention.

Another example is if I want to catch some trout on dry flies. Depending on the season, as mentioned above, and the types of insects around, I may be limited to super small midges in the 20-26 or smaller range, or be able to chuck some size 2 hopper patterns.

You can also choose to head out to the water with the intent on matching the hatches. This goes along with the above consideration about types of insects. When you choose to match the hatch, your fly size is directly affected by what you find on the water.

Choosing the Correct Fly Size Based on the Fish

Fish Caught with Barbless Hook

The last consideration to keep in mind is how the fish are acting. There are plenty of times you’ll come across some fish that are just plain difficult. You may have matched the hatch and found the perfect size fly for the bugs that are around, but the fish just aren’t feeling it.

In these cases, you may need to adjust your fly size smaller or larger to see what the fish will respond to. This one is something to keep in the back of you mind and may come into play less often. However, it does happen and can lead to many frustrating hours on the water.

Wrapping Things Up

The long and simple answer to choosing the correct fly size is that it basically comes down to what time of year it is, what type of insects are on the water, and what your goal is, and what the fish are doing. There are many other circumstances that can come into play as well, but they tend to deal more on an advanced scale when you’re talking about certain fly sizes to fish specific depths and all that fun stuff.

Keep in mind that your leader/tippet size is affected by the fly/flies you are fishing. You need to adjust this accordingly in order to get the full benefits of fishing the correct fly size. Read more about fly fishing leader and tippet for a better understanding.

With a little practice though, it will almost become second nature for you. The best thing you can do is head out on the water and try it all out to see what works and what doesn’t.

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Fly Fishing Entomology Course Available!
Mayfly Nymph Identification - Fly Fishing Entomology Course

Want to learn more about fly fishing entomology? Be sure to check out the new Fly Fishing Course on Entomology. I’ve partnered with The Catch and The Hatch to bring you this amazing resource. It will help you learn to identify the insects you encounter on the water and then put that knowledge to use in selecting the perfect fly to help you catch more fish. Check it out today!

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About Clint Losee

Clint Losee is an avid fly angler of 25+ years, web developer, and Utah Landscape & Nature Photographer. You can connect with him on Twitter, Facebook, or Google+.