Fly Tying Basics & Education

Thread Sizes: The Aught and Denier system summary

April 24, 2025

In fly dressing, thread sizes are typically indicated using an “aught” system (e.g., 3/0, 6/0, 8/0), where a higher number means a thinner thread, and a lower number means a thicker thread. The higher the number of zeros, the finer the thread. Another system, denier, measures thread weight (grams per 9,000 meters), and a lower denier generally indicates a thinner thread.

Here’s a more detailed breakdown:

Aught System:

3/0 (000): Thick, good for larger flies and saltwater patterns.
6/0 (00000): Medium-thick, versatile for various flies.
8/0 (0000000): Fine, suitable for smaller trout flies and delicate patterns.
10/0 (000000000): Very fine, ideal for midge flies and very delicate work.

Denier:

A lower denier means a lighter and thinner thread.
For example, 70 denier is a thinner thread than 140 denier.
Some threads, like UTC Ultra Thread, use denier markings.

Choosing the Right Size:

Larger flies:

Use thicker threads like 3/0 or 2/0 for stronger construction and holding power, says Fly Fishing Bow River Outfitters.

Smaller flies:

Use thinner threads like 8/0 or 10/0 for delicate work and smaller hooks.

Consider the materials you’re using:

Thicker threads might be needed for robust materials like deer hair or large hackles.

Denier can be helpful for comparing threads across brands:

If you’re comparing threads from different manufacturers, denier can be a more precise guide to thread thickness.

In summary: When selecting thread sizes, consider the fly’s size, the hook size, and the materials you’re using. A thicker thread is suitable for larger flies and stronger construction, while thinner threads are ideal for small flies and delicate work. EN-yer”) measures the linear mass density of a fiber. In simple terms, it’s the weight in grams of 9,000 meters of thread.

What is Denier?

Denier (pronounced “DEN-yer”) measures the linear mass density of a fiber. In simple terms, it’s the weight in grams of 9,000 meters of thread.

Higher Denier = Thicker, Stronger Thread: If 9,000 meters of thread weighs more, it means the thread is thicker and generally stronger.

Lower Denier = Finer, More Delicate Thread: Conversely, a lower denier means the same length of thread weighs less, indicating a finer and typically weaker thread.

For example, a 70-denier thread means 9,000 meters of that thread weighs 70 grams. A 140-denier thread would weigh 140 grams for 9,000 meters, making it twice as thick.

Denier vs. Aught System

Historically, fly dressing threads were often sized using the “aught” system (e.g., 6/0, 8/0, 10/0). This system is less precise and can be confusing because:

Inverse Relationship: In the aught system, a higher number indicates a finer thread (e.g., 10/0 is finer than 6/0).

Lack of Standardization: Aught sizes can vary significantly between manufacturers, meaning one brand’s 6/0 might be different from another’s.

The denier system offers a more standardized and consistent measurement, which is why many modern tiers prefer it.

Common Denier Ranges and Their Uses

The choice of denier depends heavily on the size and type of fly you’re dressing, as well as the materials you’re using.

Here’s a general guide:

30-70 Denier (approx. 14/0 – 8/0 aught):
- Use: Ideal for dressing very small, delicate flies like midges, tiny dry flies, and small nymphs (hook sizes 16-32).
- Benefit: Lays very flat, minimizing bulk and allowing for intricate detail without distorting the fly’s profile.

100-140 Denier (approx. 6/0 aught):
- Use: A versatile, general-purpose thread suitable for most trout flies, including standard dry flies, nymphs, and smaller streamers (hook sizes 10-16).

Benefit: Offers a good balance of strength and finesse, allowing for tighter wraps without excessive bulk.

210-280 Denier (approx. 3/0 aught and up):
- Use: Best for larger flies such as streamers, bass bugs, deer hair patterns, and most saltwater flies (hook sizes 2-10). Also good for spinning deer hair.
- Benefit: Provides significant strength to secure bulky materials, withstand aggressive strikes, and prevent breakage when applying tension.

400+ Denier:
- Use: Heavy-duty threads for very large game flies and demanding deer hair work where maximum strength is paramount.

Important Considerations Beyond Denier

While denier is a crucial factor, it’s not the only thing to consider when choosing fly dressing thread:

Material:

Nylon: Popular, versatile, strong, with some stretch that helps prevent breaking. Takes dye well for vibrant colors.

Polyester: Less stretchy than nylon but offers excellent strength and durability.

GSP (Gel-Spun Polyethylene): Extremely strong for its thin diameter. Ideal for heavy-duty applications like spinning deer hair, but can be slippery to work with. GSP threads can be significantly stronger than nylon or polyester of the same denier.

Kevlar: Another high-strength option for heavy-duty patterns, but can be abrasive.

Flat vs. Round: Some threads lay flatter (e.g., UTC Ultra Thread), which is beneficial for minimizing bulk on small flies. Others are more twisted and round (e.g., UNI-Thread), which can offer good grip.

Waxed vs. Unwaxed: Some threads come pre-waxed, providing more grip for materials like dubbing. Unwaxed threads can be waxed by the tier if desired.

Breaking Strength: While denier generally correlates with strength, the material of the thread plays a significant role. Always check the stated breaking strength if precise strength is critical for your dressing.

By understanding the denier system and considering these other factors, fly tiers can confidently select the perfect thread for any fly pattern, ensuring durability, proper proportions, and a clean finish.