Fly Fishing Fly Sizes: Choosing the Right Fly Size Instantly

One of the most challenging aspects of fly fishing is learning what size and patterns of flies to present to different fish species. It can quickly become overwhelming, especially when your local fly shop throws various numbers at you, such as size 12’s or 16’s. 

I remember being thoroughly confused when first trying to learn different fly sizes. What do all of the numbers mean? Below we will cover in-depth everything you need to know about picking the right fly, starting with how flies are sized. 

Various fly sizes laid out for observation.

How are fishing flies sized?

“Fly” size is nothing more than the size of the hook the fly is tied on. Hook sizes are determined by using a formula that measures the distance between the hook shank and the throat coupled with the length of the shaft. It is important to note that the actual sizes of the hooks will vary between manufacturers as each uses a different formula for the measurement.

Hook sizes are given in numbers starting in the middle, with size 1, and working their way outward. The hook size becomes smaller from size 1 to 32, with size 32 being the smallest hook. 

Different hook sizes in packaging for demonstration.

Where this can become confusing is for hooks larger than size 1. If the size of the hook is physically larger than a size 1, then the scale will begin to count the opposite way. Hooks larger than size 1 will be denoted with “/0” after the size. This is referred to as “aught.”

From size 1/0 to 18/0, the bigger the number, the bigger the hook. For example, a hook size of 7/0 is larger than a 5/0 hook, with size 18/0 being the largest hook.

The below graph will help give a visual representation of how hook-sizing works.

How to choose the correct fly size?

A common phrase that you will often hear in the world of fishing is “match the hatch.” While cliche, it is the most sound advice you can follow.

Fly size should be selected based on the size of the bait the targeted fish are feeding on. Flies that closely imitate prey present in the area are more likely to get eaten and increase your hook-up ratio.

For example, when fishing for trout feeding on small insects, throwing large streamer flies wouldn’t make sense. Feed the fish what they are eating and present the fly in the most natural way possible.

If the bait is on top, fish the surface with dries or topwater. If the bait is down in the water column, fish weighted flies.

Charles Hadley landscape.

Study your surroundings

When fishing in an unfamiliar area, it is best to take some time to study the environment and hone in on small baitfish, insects, and crustaceans present in the area. The term “match the hatch” is derived from different insect hatches that put trout into a frenzy—knowing what type and when these insects hatch allows you to imitate prey naturally. Applying this concept in any fishing will help you become more successful.

Have plenty of flies on hand

Having the same fly in a few different sizes allows you to move up or down size quickly to ensure you are presenting an accurate fly. If you are venturing off to unfamiliar waters, make a quick phone call to a local fly shop and ask them what types of flies and sizes you should bring before heading out on your trip.

Local knowledge is key to being successful, so seek out as many sources as you can.

Just because it is large doesn’t mean it eats large

Lastly, a common misconception is that larger flies mean larger fish. This can undoubtedly be true, but often you will be surprised by how small of flies that large fish will eat.

A good example of this is tarpon fishing in the keys. These 100+ pound giants will often sip tiny flies and ignore large bushy baitfish patterns. On the flip side, I have witnessed bass eating flies half of their body length.

Therefore there are times where fishing large baits, even for smaller fish, makes sense. Having a good variety of flies on hand will help you be prepared for the unexpected and can indeed help you catch more fish.

How fly size impacts hook size?

Fly sizes are generally referred to by what size of hook is used. Certainly, standard hook sizes are used for specific flies (especially in freshwater), but technically, there is no right or wrong formula.

When tying a fly, you should consider the hook size relative to the size of the pattern. Avoid having too big of a hook on a small fly or too small on a larger fly.

The hook should not overpower the fly, causing added weight and visibility, nor should it be too small to affect your hookset or compromise strength.

The hook Gap is key

One of the first things I do when selecting a hook is analyze the amount of gap the hook has. The gap of the hook is the space between the shaft and the hook point.

If this area is too narrow, it can cause you to lose fish due to the inability of the hook point to pierce into the fish.

If the gap is too large, the hook becomes significantly more visible to the fish, and they may not eat it.

As I design my fly, I select a hook that matches the fly’s profile and has a gap big enough relative to the amount of material on a fly. For example, a wider gap hook is best for large baitfish flies due to the amount of material between the shank and the hook point. If the gap is too narrow in this situation, the materials from the fly may prevent the hook from being able to stick into the fish.

I prefer to have the hook point slightly visible below the material of the fly. 

If given a choice, I would prefer my gap to be as large as possible. Wider gaps result in better hook-up ratios with a few exceptions.

Wide gaps aren’t always practical

Using a wide gap hook is not always practical, however. A narrower gap allows the hook to remain inconspicuous when tying small, sparse flies, such as bonefish flies. Since the amount of material on these flies is minimal, there is not much concern about the fibers or hairs blocking the ability of the hookset. 

Size 2/0 hooks up close.

When searching for wide gap hooks where you cannot see them in person, look for “2X Wide” or “3X Wide” on the packaging or description. This marking denotes a double or triple wide gap. 

Remember, the goal of the fly is to imitate natural bait as closely as possible with minimal visibility that a hook exists.

The strength of the hook

The hook chosen needs to be adequate to land the targeted species. Logically, the thinking would be the larger the hook, the stronger it will be. This is not necessarily true.

Hook manufacturers offer hooks in various diameters. The diameter of the metal of a hook is referred to as “gauge.” By offering thicker gauge hooks, flies can remain small yet strong. 

When seeking a thicker hook, look for packaging that indicates a “2x” or “3x” Heavy. The “X” means that it is two or three times as thick. Even smaller hooks are available in thicker gauges allowing you to keep flies small and strong.

A good example of this is fishing for tarpon in the keys. I will often use a size 1 hook for large tarpon because the size of the fly is small. A standard size 1 hook would likely bend out on a fish this large. Therefore a 2x gauge hook gives me the extra strength needed without sacrificing size. 

Mustad tarpon hooks in original packaging.

Why not use 2x-3x at all times?

A question I often receive is, “Why not use 2X or 3X hooks for all flies?” There are two main reasons not to use a thicker gauge hook.

The first reason is that an extra heavy hook weighs more than a single gauge hook. The hook’s added weight may cause the fly to sink too quickly or not ride correctly in the water column. The hook may be strong, but it is useless unless the fish decides to eat. 

The second reason to avoid using an extra heavy gauge is the piercing ability of the hook. Thinner diameter hooks tend to stick better in fish that have hard mouths. As the diameter of the hook becomes wider, the increased surface area requires more force to pierce the fish.

Think about the diameter of the needle when you get a shot at the doctor’s office. They are extremely thin to allow for easy entry. Now imagine if the same needle was three times as thick. It would require a harder jabbing motion to pierce the skin. 

Length matters

The shank is the long flat part above the bend of the hook. The shank is where the eyes, legs, and feathers of your fly will be attached. 

When planning out a fly design, it is imperative to make sure there is plenty of room to tie in all needed materials and that the hook point placement is correct. Longer shank hooks can be purchased, allowing more real estate for flies like baitfish patterns or large crabs. This enables the hook’s barb to be placed further back on the fly to stick any short strikes on the fly. 

Additionally, there may be times where a shorter shank hook makes sense. These can also be purchased. When tying small flies on larger hooks, the shorter shank allows you to cut the profile and weight of the hook down. 

Close shot of size 2 hooks.

Hook packaging will denote a longer shank using a “2X” or “3X” Long, whereas a short shank hook will generally be referred to on the packaging as “Xtra Short.”

Close up of Charles Hadley fish in hand.

What size fly is best for trout?

By far, the most commonly targeted fish on fly is the freshwater trout. Trout are present in various regions, which makes them easy to target, but choosing the right fly can be difficult. There are thousands of trout fly patterns in multiple sizes, so which fly and size are best?

When targeting trout, you should carry Dry Flies in sizes 10-24, Nymphs in sizes 8-22, and Streamers in sizes 6-12. Having various flies on hand will allow you to target multiple trout species, both large and small. 

Most of the time, trout will eat a well-fished fly regardless of which pattern you choose. From my experience fishing for trout, here is a list of some of the flies I always take with me:


  • Pheasant Tail in size 12 and 14
  • Gold Ribbed Hare’s Ear in size 6-18


  • Parachute Adams in sizes 12-20
  • Blue Winged Olive in sizes 16 and 18


  • Bead-Head Woolly Bugger in sizes 2-10
  • Olive Clouser in sizes 4-8

Of the flies mentioned above, my all-time favorite is an Olive Bead-Head Woolly Bugger. I enjoy fishing streamers for trout, and they have a hard time refusing this classic pattern.

Final Thoughts

Choosing the right hook can seem like a daunting task. There are quite a few inconsistencies between manufacturers in the formula used to size a hook. Your best option for selecting a hook is to think about the size of the fly you create. Matching the hook size to the fly size will serve you well.