Both wet and dry fly patterns do not come in single sizes and are frequently found in a range in order to match the natural diet of fish. In this article, we’re covering what you need to know about fly sizes, as well as provide information to help you choose the right sizes when purchasing flies, tying flies, or selecting flies while on the river.
The Importance of Fly Size Selection
Matching The Hatch
Matching the hatch and knowing your fly fishing entomology is important. Matching what fish are eating is more than just a color, in fact, the size of the fly might be the most important factor in terms of fly selection. A fly that is colored and shaped the same as what fish are currently eating still may not fool a fish if the size is off. Anglers should always be experimenting with fly sizes, as sometimes picky trout won’t take anything above a size 22, while other days, they may take just about anything.
Fly casting is largely powered by the weight of the fly line rather than the weight of the fly. However, fly casting is a sensitive art and little changes can make a big difference in how you cast. The weather can also impact this, particularly wind. Larger flies have more weight to them, which means that casting with them on windy days can help place the fly where you want it on the water. Larger flies also tend to hit the water harder, which can be bad for spooky fish, or great depending upon the conditions. Fly size selection can help your casting (or hurt it), and help the presentation of the fly.
As carnivorous fish, trout and other fly fishing targets are highly sensitive to the way insects land on the water. Your casting and mending techniques play a big role, but so too does the fly itself. Your fly doesn’t just need to mimic an insect– it needs to have the appropriate size, be drifting appropriately, and also be located in the right place of the water in order to look right to the fish.
Big flies can land harshly on the water that can attract fish in certain waters, or spook them depending. In addition, larger flies can be more buoyant, which can be beneficial in certain situations. Fishing around spooky fish may require a reduction in fly size, while fishing a larger fly may be necessary to entice action in other situations.
Fly location is a vital consideration when nymphing or using wet flies in any way and can inform your presentation. Dry flies stay on top of the water, but you’re putting nymphs in the water column. You need to get them in the right part of the water to be appealing to fish, and this can be a challenge for smaller flies in fast-moving water.
Larger wet flies tend to be heavier, and sink faster, while smaller flies may need the help of other flies or split shot in order to sink quickly. Therefore, fly size does effect fly location in the water. Larger flies also experience more water resistance, and therefore can be affected by currents more than a smaller fly of the same weight.
Considerations When Choosing the Correct Fly Size
There are lots of things to consider when choosing fly size. Here are some of the primary considerations for anyone who’s making a decision about which fly sizes to use and when to use them.
Time of Year
Insects have a short, seasonal life cycle. Sometimes matching the hatch isn’t an option because there isn’t a hatch happening. For instance, very few temperate regions have any insect hatches in winter. This means that the majority of insects available are midges and other small insects. If you’re fishing dry flies in winter, flies size 18 and under may the most appropriate. If you’re nymphing, you can go a little larger with scud or caddis larvae patterns, because these are always present in many waterways.
In warmer months, you can fish larger flies and use a greater variety of patterns with better success. Summer months are more likely to see terrestrial insects get washed into waterways, so these flies are often successful. Insects often reach their full mature size in summer, so larger flies may present better to fish in summer. A good rule of thumb is that colder weather means smaller flies, and warmer weather means larger flies (though small flies tend to work year round).
Note: Year round, in most regions, smaller flies than the ranges presented above will still work well.
In addition to general seasonal changes, daily temperature changes can affect the type of fly you want to use. You need to pay attention to the water temperature to see how the fish and insects are going to act and choose fly sizes accordingly.
As a general rule, if the water is hotter than 67º F, you shouldn’t be fly fishing. The fish are too exhausted and stressed due to lack of oxygen in the water at these temperatures. For water between 65º and 66º, you should “rope up,” or use a larger, heavier tippet to bring the fish in more quickly to reduce stress and to increase their chances of survival after release. When you increase the size of the tippet, you also need to increase the size of the fly. Tippets and flies are sized similarly– the higher the number, the smaller the size in both cases. For instance, a 4x tippet often won’t fit through the eyelet of a size 20 fly, therefore, anglers will need a smaller tippet, or a larger fly.
Tippet and Fly Size Chart
Here’s a chart that will help you choose which fly size to use with your larger tippet.
|Tippet Size||Tippet Diameter||Fly Size|
|8X||0.003”||28, 26, 24, 22|
|7X||0.004”||24, 22, 20, 18|
|6X||0.005”||22, 20, 18, 16|
|5X||0.006”||18, 16, 14|
|4X||0.007”||16, 14, 12|
|3X||0.008”||10, 8, 4|
|2X||0.009”||8, 4, 6|
|1X||0.01”||6, 4, 2|
|0X||0.011”||4, 2, 1/0|
For extended tippet sizes and more information, consult our Tippet Size Chart article.
Trout have acute vision and are very visual hunters. If there’s high turbidity and the water is murky, a larger fly might be easier for the fish to see. Water conditions can also favor smaller flies. Ultra clear waters mean that trout can see clearly, this often means that you simply have to use small flies to trick them.
In water with lots of obstacles and snags, you may want a smaller fly that’s less likely to get caught on hazards like logs and other debris.
When the water in your favorite trout stream gets low, you can still go fly fishing. Fly sizes do need to change from the typical ones you’d use when the water levels are higher, because the fish are typically found in different places in the stream, and because they will spook more easily. You will be more likely to see them, and they can almost definitely see you. So you need to present carefully, and you need to offer something really tempting to break through that wariness.
When the water level is low, fish will congregate in the deepest pools, but they’re often resting, not hunting. This is because as the water level drops, the water temperature often gets higher, and trout prefer to exert energy in cooler water. Low waters often means trout can spook easier, and it often means the water is clearer, therefore, smaller flies are often needed for low water situations.
Fish are animals, and animals are frequently unpredictable. Fish don’t operate on anybody’s schedule, and so sometimes you need to be flexible about how you lure them. If the fish aren’t biting, a bigger fly might entice them, so might a smaller one. Experimentation is key.
Additionally, the trout’s visual acuity is geared towards movement. There’s no hard and fast rule for sizes, but if the fish aren’t biting, a fly that looks a little bit different from the rest of the insects on the water can catch their attention. You can go a little bigger or a little smaller- either way, changing the size of your fly can create a visual disruption that triggers a reaction strike from the trout.
Anglers that are changing flies, depths, and presentations regularly until they start catching fish are the anglers that catch the most fish.
Depending on what you are trying to mimic, you might want to choose a smaller or larger fly for patterns that look like multiple types of insect or are attractor patterns that don’t mimic anything in particular. Many fly patterns can be used with multiple species– the beloved Wooly Bugger, for instance, is a streamer that can remind trout of a baitfish, a soft-bodied annelid or leech, or any number of other tasty treats.
Depending on the size of the fish you’re after, you may choose a smaller or larger fly. Bigger flies often draw bigger fish, but if a meal is too big, the fish might not go for it.
The rule of thumb here is to feed the fish what they are eating. When you’re fishing for trout that feed on small insects, don’t offer them a large streamer. Present your fly in the most natural way possible to convince the fish that it’s something they want to eat.
Understanding Fly Sizes
To tie a fly, fur, feathers, and other natural and synthetic materials are wrapped around and tied to a hook. Depending on techniques, two flies can be the same size but may look quite different. For example, a fly that’s tied with the parachute technique, which creates a big area that looks like a wing, is going to look larger than a tightly tied nymph on the same hook size.
Small, medium, and large are all relative terms when it comes to flies, so the easiest way to talk about them and categorize them is by the standardized part that doesn’t change no matter what the pattern looks like: the hook.
Depending on what kind of fly fishing you do, hook type and hook features may be very important. If you’re going for huge saltwater fish like tarpon, you have to make a lot of choices regarding the tradeoffs of increased hook gauge and gap size being more visible to fish but also making it more likely that the hook doesn’t break. For freshwater fish like trout, the breaking of a hook isn’t much of a concern, but the overall presentation is of great concern.
Hooks, just like tippet and leaders, are sized like wire gauge in that the bigger the number, the smaller the hook. The size is determined not by diameter or actual length of the hook, but rather by the length of metal used to make the hook. This means that sometimes you’ll see an x-rating (2xl, 4xl, 6xl, etc) scale on hooks that have longer or shorter shank sizes.
This affects fly sizes based on how much hook area you have for tying and creating the body of the fly. If you tie your own flies, you might want to experiment with some XL hooks to create some intermediate-sized flies. Remember, you’re mimicking natural animals, and no two insects are exactly the same. It’s always a good idea to have a variety of fly sizes with you, and it’s fun to experiment!
What Sizes of Fly Do I Need?
When targeting trout, you should carry dry flies in sizes 8-24, nymphs in sizes 8-28, and streamers in sizes 6-12. Having various fly patterns and sizes means that you have a greater variety of options for the trout. Think of it like offering a bigger “menu.” It’s always better to have more flies than you need at any given time than to miss out on fish because they aren’t taking what you have to offer.
While the fly patterns you choose are entirely up to you, here’s a sample loadout that would likely bring success on most trout streams. All of these patterns are on our list of essential flies and are popular because of how successful they are– and they’ll fit in a 50-fly fly box perfectly.
- Pheasant tail in sizes 10-20
- Hare’s Ear in sizes 8-18
- Killer Bug in sizes 10-14
- Dry Flies
- Elk Hair Caddis in sizes 10-20 (or more if wanted– fish love this one)
- Parachute Adams in sizes 10-18
- Royal Wulff in sizes 8-16
- Wooly Bugger in sizes 2-18
- Clouser Deep Minnow in sizes 4-10 (go larger if you’re fishing really deep water)
- Muddler Minnow in sizes 2-12
Fly Size Chart
Now that you know the reasons to have flies in multiple sizes, here’s a quick reference to help eliminate some of the guesswork.
|Tippet Size||Fly Size||Fish|
|8X||20-28||Small trout, panfish|
|7X||18-24||Easily spooked trout|
|6X||16-22||10-12 inch trout|
|5X||14-18||12-20 inch trout|
|4X||12-16||12-20 inch trout|
|3X||8-12||20+ inch trout, smaller bass|
|0X||1/0-4||Saltwater game fish|
Note: The chart above is general ideas, there is definitely grey area for all of the above categories.
The Final Word on Fly Size
Fly fishing is an incredibly situational sport. There’s no universal answer to which fly size you should be using, but for every fishing situation, there’s some guidance to be found. Fly fishing is an art and a science, and you should always be willing to make changes as you go, but this information will help you get started and serve as a great point of reference.
Max DesMarais is the founder of hikingandfishing.com. He has a passion for the outdoors and making outdoor education and adventure more accessible. Max is a published author for various outdoor adventure, travel, and marketing websites. He is an experienced hiker, backpacker, fly fisherman, trail runner, and spends his free time in the outdoors. These adventures allow him to test gear, learn new skills, and experience new places so that he can educate others. Max grew up hiking all around New Hampshire and New England. He became obsessed with the New Hampshire mountains, and the NH 48, where he guided hikes and trail runs in the White Mountains. Since moving out west, Max has continued his frequent adventures in the mountains, always testing gear, learning skills, gaining experience, and building his endurance for outdoor sports. You can read more about his experience here: hikingandfishing/about