3 Things to Consider When Choosing Fly Tying Thread

When choosing the best thread to use for a fly pattern, there are 3 main characteristics to understand: material type, size, and flat-ability. Of course the color of the thread is also very important, but once you narrow down your search by these 3 characteristics, you can choose the best color available in that type.

Material Type

There are several different types of material that fly tying thread is commonly made from. Silk was one of the earliest materials used but has mostly been replaced with more modern alternatives. Silk thread is not very strong and it doesn’t hold color very well when wet. Newer materials improve on both of those aspects.

Nylon is used in many popular thread types. Danville Flymaster is one of the most well known nylon thread types. Nylon thread typically has about 25-30% stretch, it lies flat well, and has a glossy sheen to it. One of the downsides of the glossiness of nylon thread is that dubbing slides on the thread quite a bit. This does not mean you can not use dubbing on nylon thread, it is just a little more difficult to work with.

Polyester is used in many popular threads such as UNI-Thread and Veevus thread. Polyester is stronger, heavier, and has less stretch than nylon (about 15%). It is not as glossy though, so the color tones tend to be more muted than nylon threads. Because polyester is less glossy, and has a rougher texture, it holds on to dubbing better than nylon. Polyester threads are stronger than nylon, so you can get the same breaking strength in a smaller diameter thread. In general, polyester is better for grabbing and biting on materials that need to be secured to the hook.


Understanding thread size is important when choosing the best thread for a given fly pattern. The first thing to understand is that there is no standard among thread manufacturers to denote thread size. Originally the Danville Chenille Company starting using the aught system, which was used for sizing silk surgical sutures. The aught system gives a value in a number of zeros. 3/0 (pronounced “three aught”) means three zeros, or 000. 6/0 = 000000, 8/0 = 00000000. The more zeroes, or aughts, the smaller the thread size. This system is still used by thread manufacturers today, but it doesn’t tell us anything about thread size of one brand compared to another.

A more universal way to help determine thread size is the denier system. Denier is the weight, in grams, of 9,000 meters of thread. It is the actual weight of the thread and therefore can be used to universally compare threads across brands. 70 denier, 140 denier, and 210 denier are common thread sizes/weights. It’s easy to understand since it is weight; 140 denier is going to be a bigger/heavier than 70 denier. Most manufacturers who measure their thread by the aught system will also tell you what the denier rating is. In this photo you can see that the UNI-Thread 3/0 is 220 denier, the 6/0 is 136 denier, and the 8/0 is 72 denier. One thing to note though is that denier and thread thickness do not correlate.

aught and denier example


Depending on the pattern you are tying, you may prefer the thread to lay flat on the hook, or have a more rounded profile. Flies that require a smooth underbody are better tied with thread that can lay flat well. You can build bodies with a smooth texture and low bulk with no ridges. On the other hand, rounder thread is good for grabbing different materials and securing them to the hook. If you are tying with materials such as deer, elk, or moose hair, a round profile thread will grab and bite down much better than flat profile threads. We have included a chart of the threads that we carry to help you choose the best thread for your needs.

Some other specialty threads

UNI-Floss is a single strand floss made of 100% rayon. Rayon is very shiny and strong. Great for laying flat body wraps with a high gloss. Splits really well for various applications. Some common uses are the Royal Wulff, and salmon fly bodies.


A fine elastic strand with a unique texture for wound bodies or legs. It can be used as a replacement for floss, and wound with a variety of tension levels to reduce or eliminate slippage. Used as leg material, the flat shape generates more movement in the water. The strand can be divided for even smaller applications. 15 yds. per spool.


Midge Body Thread
Midge body thread is a unique thread that has a textured and shiny quality. There is a main thread running through the middle of the textured shiny material to give the thread strength and functionality. While it is excellent for tying midge bodies, you can let your imagination go and come up with lots of different uses for this thread.

Midge body thread

midge body thread - midges
Some Zebra Midges tied with the MFC Midge Body Threads

GSP Thread
GSP stands for Gel Spun Polyethylene. GSP thread is super strong for it’s size, very slick and has very little stretch (about 3%.) GSP is usually used for spinning deer hair or synthetic materials that need a very strong and thin thread. GSP is not good for dubbing because of it’s slickness.

Monofilament Thread
Monofilament thread is basically single strand fine nylon tippet-like material. It is clear and can not be seen when covered with head cement or epoxy. It’s good for creating a natural looking segmentation on scud bodies too. It’s not very strong compared to tying threads and it is prone to over twisting during the tying process.

Comparing one brand of thread to another is pretty difficult. Since there are no standards between manufacturers, it is best to take in to account all of the characteristics of the thread, and choose the best type for the fly pattern you are tying. For the majority of tying that I do (rocky mountain trout patterns,) Veevus threads are the most versatile. They are very strong compared to UNI-Thread and can be flattened out or corded up based on the needs of the pattern. I encourage you to experiment with different threads. You will find that different threads all have strong suits that will perform better with different tying needs. This chart is designed to help you understand the differences between the threads that we carry. As we add more thread selection to our inventory, I will update this chart with the new data.

Happy tying!

Thread Size Denier Material Profile
Breaking Strength
UNI-Thread 3/0 220 Polyester Round Poor 0.91 kg
UNI-Thread 6/0 136 Polyester Round Poor 0.93 kg
UNI-Thread 8/0 72 Polyester Round Poor 0.45 kg
Veevus 6/0 110 Polyester Flat Great > 1 kg
Veevus 8/0 110 Polyester Flat Great ~ 1 kg
Veevus 10/0 110 Polyester Flat Great 0.8 kg
Veevus 12/0 70 Polyester Flat Great 0.53 kg
Veevus 14/0 70 Polyester Flat Great 0.52 kg
Veevus 16/0 50 Polyester Flat Great 0.43 kg
Danville Flymaster + 140 Nylon Flat Great 0.91 kg
Danville Flymaster + 210 Nylon Flat Great 1.36 kg
Danville Flymaster 6/0 70 Nylon Flat Great 0.31 kg
UNI-Floss 600 Rayon Flat Great
Danville Monofilament Fine Mono Nylon Round Terrible
Midge body thread 230 Round Poor

7 thoughts on “3 Things to Consider When Choosing Fly Tying Thread

  1. Very nice article, one thing I still don’t understand is the Danville letter coding for size A,B, D AND E. Can you explain these size codes thanks.

  2. Something you might want to mention is that with a “flat” thread, you can use it either as a flat or round thread, so you don’t have to buy both. As you tie with a flat thread, it gets twisted and will no longer lie flat. You have to give your bobbin a quick spin to untwist it. Likewise, you can spin in the other direction to make it round if that’s what you. That’s why I always tie with Ultra Thread, but I’m sure Veevus or any flat thread would be the same. Also you can use the split thread dubbing technique with flat threads and create less bulk than with a round thread dubbing loop.

  3. My husband loves fishing and I would love to get him some fly fishing materials for Christmas but I don’t know much about it, so I am glad that I found this article. You make a great point that polyester is a very strong and heavy thread and I think that this would be great for my husband because this way the flys that he uses will be more durable. Also, it is good to know that round thread is good for securing different types of materials on your hook and this would be a benefit for my husband because he loves using a lot of different materials each time he goes to catch fish.

    1. We are glad you liked the article. There is definitely a lot to picking a thread so we’re glad it helped. Let us know if you have any other questions.

  4. Danville is my preferred, with Veevus as a second choice. Uni thread is unique in the color variety, that Veevus is short on in the various sizes. Depends on what pattern, and what’s being secured to the hook that drives my preference. I had stopped tying for 30 years and picked it back up in the past few years and my Danville threads have lasted that long and are great to tie with, still.

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