Fly Tying Thread Guide: Types, Sizes & Pro Tips for Beginners
With: Lindsay Simpson Fly Fishing
Understanding fly tying thread is essential for tying effective, durable flies—this guide breaks down thread types, sizing systems, and essential tips every fly tyer should know.
Fly Tying Thread Basics
Learn the Main Types of Fly Tying Thread
Nylon: Stretchy, glossy, general-purpose thread. Easy to dye, smooth finish.
Polyester: Stronger than nylon, minimal stretch, slightly duller colors.
Gel Spun Polyethylene (GSP): Super strong, slippery, low stretch—great for large flies or deer hair work.
Silk: Traditional, natural finish, rarely used today except for classic patterns.
Cotton/Wool: Natural fibers, limited modern use.
💡 Pro tip: You can switch threads mid-fly depending on the technique—e.g., use GSP for spinning deer hair.
Understand Thread Sizing Systems
"Aught" (O) System: Lower numbers = thicker thread
18/0: Ultra fine (for tiny midges)
8/0 – 6/0: Standard for small/medium trout flies
3/0: Heavier thread for larger flies
1/0 – 2/0: Extra heavy, ideal for saltwater and big streamers
Denier System: Based on fiber weight
50 Denier: Light, versatile for trout flies
100 Denier: General purpose, good strength
200 Denier: Thick, ideal for streamers or saltwater flies
📏 Denier = grams per 9,000 meters of thread. Higher number = thicker thread.
Choose the Right Thread for the Job
Nylon (25% stretch): Ideal when flexibility is needed
Polyester (15% stretch): Balanced strength with low elasticity
GSP (3% stretch): Perfect for strength-intensive tying (e.g., spinning hair or saltwater patterns)
Know the Key Thread Attributes
Strength: Nylon & polyester are strong; GSP is ultra-strong
Finish: Silk offers a beautiful classic finish; synthetics are more durable
Colorfastness: Ensure bright colors won’t bleed when wet
Fray Resistance: Silk can fray more easily—choose synthetic threads when durability matters
Money-Saving Thread Hack 💰
Buy only white GSP thread in 50D and 100D—cover almost all tying needs
Use permanent markers (like Pro Markers or Sharpies) to color your thread as needed
Great for creating colored heads without stocking dozens of spools
Keeps your fly tying kit minimal and budget-friendly
Thread Flattening & Splitting Trick
Twist your bobbin counterclockwise to flatten the thread
Useful for split-thread dubbing techniques
Twist clockwise to retighten the thread for finer wraps or stronger hold
