I constantly get asked questions about fly tying thread sizes and what these variations mean. And with good reason too. It is just bloody confusing at times.

There is nothing like an industry standard to keep everything in line. Except when you have two, and one is more of a guideline than an exact measurement, and you’re measuring different materials based on one (or two) systems. So it goes without saying that many of you would be right to be a little confused when trying to decide which thread to use for any given application.

There are two main measurements used in fly tying for to delineate thread sizes. These are the old Aught measurement and the more modern Denier rating. Both are a perfect system when assigned to one material, from one brand. It is when we look at how different brands assign the rating, or vary their thread material that it all gets a little confusing.

The Aught rating

This is shown in a number of common sizes including 3/0, 6/0 and 8/0. More modern threads with greater breaking strain are now available in sizes that include 12/0, 14/0 and 16/0. This is often a source of confusion as one assumes that the higher the number preceding the slash, the thicker the thread will be. But it is quite the opposite.

The easiest way to look at the Aught system is to think of zero as the baseline thread. Then additional zeros take you thinner and thinner. so 3/0 would be one third of the thickness of the mythical zero baseline. 6/0 would then be twice as thin, and 8/0 even thinner still. Once you think if it in this manner, it is pretty simple. But, there is no industry standard as to what that baseline zero should be for thickness, so every manufacturer has can have variations in what thread is rated.

FLY TYING THREAD SIZES EXPLAINED AT THE TROUTLORE FLYTYING STORE

8/0 breakdown

Let’s look at a couple of the 8/0 threads that we stock.

UNI 8/0 thread is is a waxed polyester that lays fairly flat on the hook, but still with a little bit of a bump. This thread has a breaking strain of approximately 16 ounces of pressure. This is about the same strength as 6/0 Danville’s Flymaster thread, which is a thicker thread.

Veevus 8/0 thread is a two-strand polyester thread that can be split easily for dubbing loops and offers very little bulk on the thread. This thread has a breaking strain of up to 32 ounce of pressure. This is twice that of the UNI 8/0 and more than the breaking strain of 3/0 Danville’s Mono thread!

So, as you can see, the Aught measurement can offer an idea of strength and size. But only within one brand and one material.. It is not a perfect measurement across brands. There can be big variety in the materials used and the construction of the thread itself. If you stick to one brand for the most of your threads, it’s easy. The the Aught system gauges that thread you’ll be using for various uses. But it is not a true indicator of size or strength of the thread itself.

Denier

Now, let’s look at the Denier measurement. This was first introduced to fly tying threads in the 90s when the Wapsi company brought out their UTC Ultra-thread. Many of you will have tied with this thread for a long time, I know I have. And you’ll all know the “D” that is associated with their thread thicknesses. But what does it represent exactly?

Denier is a measurement that has been used in the textiles industry for a very long time. It refers the the weight of 9000 meters of a thread in grams. So, if you happen to have exactly 9km of thread laying about and some accurate scales you can easily determine the denier of that thread. Otherwise, just leave it to the manufacturer to specify.

This measurement is far more consistent than the Aught rating as it determines the weight of the thread, and henceforth the thickness. But, 70 denier silk is going to be much finer than 70 denier nylon, but not necessarily stronger, purely based on the material’s weight.

The common fly tying thread sizes many of you will see are 70 Denier, 140 Denier and 210 Denier. These can be loosely equated to 8/0, 6/0 and 3/0 thread respectively, but not always. UNI now print both the Aught and Denier rating on their thread spools, with their 8/0 thread weighing in at 72 Denier. That 2 grams of extra weight over 9 kilometers manages to increase the breaking strain by upwards of 3 ounces compared to the UTC 70 Denier.

Wire

To further confuse matters in fly tying thread sizes, let’s take a very quick look a wires. The wires on offer are also thrown in with a range of different measurements. Although in this instance, most are actual understandable values. UNI Dubbing Brush Wire for instance is measure in thousandths of an inch. Even this doesn’t make complete sense because it is mixing imperial and decimal measurements to create a value. For all the guitarists out there it will instantly have some guilded logic. The .009 wire is 9 thousandths of an inch thick, or the equivalent of a high E string on a Strat. At least, in standard tuning, when it comes out of the factory. And before it has been change to the player’s liking.

Still confused?  It gets worse. Wapsi derived a simple label system for their UTC brass wire that no one could get confused with. Here you have Extra Small, Small, Medium, Large and another size called Brassie. But is Brassie bigger than Medium because I tie some big brassie flies. Or is it thinner than Small since a lot of people tie tiny brassie flies? Actually, it sits in between Small and Medium, obviously. This is a little less clear than their Lead and Lead Free Wire. That reverts back to the decimal place in and inch measurement.

So…

In short, there are no real hard and fast rules for thread and wire measurements in the fly tying industry. Different manufacturers adopt different standards. Sometimes several different standards are found across similar products. We are left to decide what we like to use. It all makes sense when you have been tying with a certain thread for a while and you know the feel and how it behaves on the hook. However, it can be frustrating when you don’t have them all on hand just to try out.

I hope this has been more informative than it has been confusing. I am continuing this series of blog posts to help answer the frequently asked questions I get relating to fly tying materials. You can find more of these posts in the Fly Tying category of our blog page. Plus, you can keep up to date with new releases through our newsletter and social media outlets.