Fly Tying Thread Sizes: Which Size for Which Fly

Choosing the right thread size is a critical aspect of fly tying. The right thread can significantly influence the quality and durability of the finished flies. Fly tying thread sizes impact the strength, bulk, and detail of the fly being created. Finer threads are often preferred for delicate patterns like midges, where minimal bulk is crucial for realism in presentation. Conversely, for larger streamers or saltwater flies, heavier threads provide the necessary strength to handle tough conditions and large fish. Once you've mastered thread selection, you'll elevate your fly tying skills and create more effective flies for catching fish.

Understanding Thread Sizes

Fly tying thread isn’t just a simple spool of fiber—it plays a crucial role in the durability, proportion, and overall appearance of your flies. Choosing the right thread size ensures you can secure materials effectively without adding unnecessary bulk or risking breakage. However, with different sizing systems and varying thread strengths, understanding how to select the best option for each fly pattern can be confusing. It’s important to consider the difference between the denier and aught measurement systems and how to balance strength and diameter for the best results. You might also consult a  thread comparison chart for extra help.

Thread Sizing Systems – Denier vs. Aught

When shopping for fly tying thread, you’ll often see two different sizing systems: denier and aught ("/0") ratings. While both measure thread size, they do so in different ways, which can be confusing. Understanding these systems will help you select the right thread for your fly patterns with confidence.

Denier

This system measures the weight of 9,000 meters of thread in grams. A higher denier number indicates a thicker, stronger thread, while a lower denier number means a finer, lighter thread. For example, a 70-denier thread is thinner than a 210-denier thread.

Consider the following Denier ranges when choosing fly tying thread sizes for your projects:

• 70-100 Denier: Fine threads for small, delicate patterns

• 140-210 Denier: Medium threads for most trout flies

• 280-300 Denier: Thicker threads for larger flies and saltwater patterns

• 400+ Denier: Heavy-duty threads for big game flies and deer hair work

Aught ("X/0") Rating

This older system is based on the diameter of the thread. It's a rough scale, with higher numbers indicating finer threads and smaller numbers indicating thicker threads. A 14/0 thread is much finer than a 6/0 thread. However, thread ratings aren’t standardized across brands, so a 6/0 thread from one manufacturer might be slightly different from another. Silk and Kevlar threads are often measured using this method.

Here's a quick reference for common Aught sizes:

• 3/0 (000): Thick, suitable for large flies

• 6/0 (00000): Medium, versatile for many patterns

• 8/0 (0000000): Fine, ideal for small trout flies

• 10/0 (000000000): Very fine, used for delicate patterns

While aught ratings are still common, denier is the more precise and reliable measurement since it provides an actual weight-based size. If you’re comparing threads across brands, checking the denier will give you a clearer idea of thickness and strength. However, many fly tyers still prefer the aught system because it’s familiar and widely used in traditional fly recipes.

By understanding both systems, you can make better choices when selecting thread for specific fly patterns—whether you’re tying delicate dry flies or robust streamers.

Strength vs. Diameter – What Matters Most?

When you're selecting fly tying thread sizes, two key factors come into play: strength and diameter. A thicker, stronger thread might seem like the best choice, but the right balance depends on the type of fly you're tying and the materials you're working with.

• Thread Strength: Measured by denier or breaking strength, stronger threads allow for tighter wraps and better durability. This is important for flies that need to withstand aggressive strikes, such as streamers or saltwater patterns. However, excessive thread strength can crush delicate materials or create bulky wraps.

• Thread Diameter: A finer thread (lower denier or higher aught rating) lays flatter and is ideal for intricate flies, such as dry flies and nymphs, where excess bulk can affect buoyancy and realism. Thinner threads also allow for more wraps without excessive buildup, making it perfect for detailed patterns.

Finding the Right Balance

The best choice depends on the fly pattern and the materials you're using. For delicate dry flies, a fine thread like 14/0 or 70 denier is ideal to prevent bulk. For larger flies, such as streamers or bass bugs, a stronger thread like 210 denier ensures durability without breaking under pressure.

In the end, choosing between strength and diameter depends on the specific needs of your fly pattern. If in doubt, consider the materials you're tying with and the size of the hook—these will guide you toward the best thread for the job. Analyze a fly tying thread size chart to choose the right thread for your flies.

Practical Applications of Thread Sizes in Fly Tying

Choosing the Right Thread Sizes for Different Fly Types

Selecting the right fly tying thread size is essential for creating durable, well-proportioned flies suited to different angling conditions and target species. The thread you choose impacts not only how the fly looks but also how well it performs on the water. Different fly types require different thread sizes to accommodate materials, maintain proportions, and ensure durability.

Best Fly Tying Thread Sizes for Dry Flies

Dry flies require thin, lightweight thread to maintain a natural profile and buoyancy. A thicker thread can add unwanted bulk, making the fly appear unnatural or causing it to sink prematurely.

• Recommended sizes: 14/0 to 8/0 (or 30-70 denier)

• Why? A fine thread allows for precise, delicate wraps without overwhelming the fly body.

• Examples: Adams, Elk Hair Caddis, Parachute Mayfly

A 14/0 or 16/0 thread is best for small dry flies (#18 and smaller), while 8/0 or 10/0 provides more strength for slightly larger patterns. Choosing a flat-laying thread can also help reduce bulk while ensuring a secure tie.

Best Thread Sizes for Nymphs and Wet Flies

Nymphs and wet flies are often tied with slightly stronger thread than dry flies because they require durability and sometimes added weight to sink effectively.

• Recommended sizes: 8/0 to 6/0 (or 70-140 denier)

• Why? A balance between strength and diameter is needed to secure materials like dubbing, wire, and hackle while keeping the fly profile slim.

• Examples: Pheasant Tail Nymph, Hare’s Ear Nymph, Soft Hackles

For small nymphs (#18 and smaller), 8/0 (70 denier) is ideal. For medium to larger nymphs and wet flies, a 6/0 (140 denier) thread provides extra strength without excessive bulk.

Best Thread Sizes for Streamers and Large Flies

Streamers, bass bugs, and saltwater flies demand strong, heavy-duty thread to hold materials like bucktail, synthetics, and large feathers in place.

• Recommended sizes: 6/0 to 3/0 (140-210 denier and up)

• Why? These flies undergo aggressive strikes, so a thicker, more durable thread prevents breakage.

• Examples: Woolly Bugger, Clouser Minnow, Deceiver

For medium-sized streamers, 6/0 (140 denier) is a solid choice. For larger predator flies and saltwater patterns, 3/0 (210 denier) or stronger is preferred to handle tough materials and hard-fighting fish.

By matching the right fly tying thread size to your fly type, you’ll ensure stronger, more effective patterns that perform as intended on the water.

Best Thread Sizes for Freshwater vs. Saltwater Flies

The environment you’re fishing in plays a big role in selecting the right fly tying thread sizes. Freshwater and saltwater flies have different demands when it comes to durability, material choice, and overall thread strength.

Freshwater Fly Thread Guide

Freshwater flies, particularly dry flies, nymphs, and small streamers, require finer thread to maintain a natural appearance and avoid unnecessary bulk. Since most freshwater species, like trout and bass, aren’t as aggressive as saltwater fish, you can use lighter threads without worrying about breakage.

Saltwater Fly Thread Guide

Saltwater flies need to withstand harsh conditions, tough fish, and aggressive strikes. They’re often tied with thicker, high-strength thread to secure large materials like bucktail, synthetic fibers, and epoxy coatings.

Since saltwater flies often require extra durability, waxed threads and synthetic materials like gel-spun polyethylene (GSP) are popular choices. These materials offer high strength without excessive bulk.

By selecting the right fly tying thread sizes for freshwater or saltwater patterns, you’ll create flies that not only look great but also hold up against the fish you’re targeting.

Frequently Asked Questions About Thread Sizes

Can I use the same thread size for all parts of a fly?

While it's possible to use the same thread size throughout a fly, it's not always ideal. Different parts of the fly may require varying thread thicknesses for optimal construction and durability. Matching fly tying thread sizes to the specific fly pattern and materials used ensures better proportions and overall quality.

How can I prevent thread breakage when tying flies?

To prevent thread breakage when tying flies, use high-quality thread, wax it for strength, and maintain consistent tension. Avoid overtightening, especially near the hook eye. Use a bobbin with smooth thread delivery and handle delicate materials gently. Practice proper thread management techniques for durability.

What materials are fly tying thread commonly made from?

Fly tying thread is commonly made from a variety of materials and textiles, each offering different strengths and characteristics suited for specific fly patterns. The most popular are nylon, polyester, and GSP.  Danville thread has several options available.

• Nylon – A popular choice due to its durability, stretch, and smooth finish, making it ideal for both beginners and advanced tyers.

• Polyester – Strong and slightly less stretchy than nylon, providing good color retention and resistance to UV degradation.

• Silk – Traditionally used for classic and delicate fly patterns, particularly in wet flies and salmon flies.

• Cotton & Wool – Less common in modern fly tying but sometimes used in traditional patterns for added texture and absorbency.

• Kevlar & GSP (Gel-Spun Polyethylene) – Ultra-strong threads used for large flies, saltwater patterns, and flies requiring extra durability.

How can I enhance my fly tying thread?

There are several ways to improve the performance of different types of fly tying thread to make them more durable, easier to work with, and better suited for specific fly patterns:

• Waxing – Applying wax to your thread improves grip, reduces slippage, and helps secure materials like feathers and fur more effectively.

• Dubbing Wax or Resin Coating – Adding dubbing wax or resin strengthens the thread and makes it more resistant to fraying, which is ideal for delicate or intricate flies.

• Pre-Stretching – Lightly stretching the thread before use can help prevent breakage and create a smoother, tighter wrap.

• Color Customization – You can dye your thread to achieve custom colors for better pattern matching and to attract specific fish species.

• Using Multiple Threads – Doubling up finer threads or twisting threads together can increase strength while maintaining a thin profile.