Fly Tying Thread Sizes Explained: Complete Guide to 6/0, 8/0, 3/0
Thread sizing in fly tying can be confusing, with different systems and seemingly backwards numbering. This guide breaks down everything you need to know about choosing the right thread.
Quick Answer: What Thread Size Should I Use?
- Small flies (size 18-24): 8/0 or 12/0 thread
- Medium flies (size 10-16): 6/0 or 8/0 thread
- Large flies & streamers (size 2-8): 3/0 or 6/0 thread
- Saltwater flies: 3/0 or flat waxed nylon
Understanding the Aught System (3/0, 6/0, 8/0)
The aught system uses a slash-zero format where higher numbers mean thinner thread. This seems backwards, but it follows the same logic as fishing line and wire gauges.
- 3/0 - Thick, strong thread for large flies and deer hair work
- 6/0 - The most versatile all-round thread size
- 8/0 - Fine thread for small flies and delicate work
- 12/0 - Very fine for midges and tiny patterns
- 16/0 - Ultra-fine specialty thread
Denier: The More Accurate Measurement
Denier measures the weight of 9,000 metres of thread in grams. Unlike aught sizing, higher denier numbers mean thicker thread. Many modern thread manufacturers now list denier alongside traditional sizing.
| Aught Size | Denier (approx) | Best For |
|---|---|---|
| 3/0 | 200-280 | Saltwater, deer hair, large streamers |
| 6/0 | 130-150 | All-purpose, most trout flies |
| 8/0 | 70-80 | Small dries, emergers, nymphs |
| 12/0 | 40-50 | Midges, size 20 and smaller |
| 16/0 | 20-30 | Ultra-small patterns, competition flies |
Thread Materials Compared
Polyester Thread
The most common type of fly tying thread. Polyester is strong for its diameter, takes dye well, and works for most applications. It can be waxed or unwaxed.
Nylon Thread
Slightly more stretch than polyester, which can be helpful when working with delicate materials. Some tiers prefer it for flattening out when wrapping.
GSP/Gel-Spun Thread
Incredibly strong for its diameter. GSP thread is useful for spinning deer hair and applications requiring maximum strength. It can be difficult to control and cuts materials easily.
Common Thread Sizing Mistakes
- Using thread that is too thick: Builds up bulk quickly, makes small fly heads oversized, and can crack hook eyes on small hooks.
- Using thread that is too thin: Breaks frequently under tension, struggles to secure bulky materials, and makes building thread bodies tedious.
- Ignoring brand differences: A 6/0 from one manufacturer may not match another. Check the denier for accurate comparison.
Thread Recommendations by Fly Type
- Dry flies: 8/0 for most sizes, 12/0 for size 18 and smaller
- Nymphs: 6/0 or 8/0 depending on size
- Streamers: 6/0 for small, 3/0 for large articulated patterns
- Bass bugs: 3/0 or GSP for spinning deer hair
- Salmon flies: 6/0 for most, 8/0 for low-water patterns
Track Your Thread Collection
Keep track of your thread inventory, compare sizes across brands, and never run out mid-pattern. Woolly Bug helps you organise all your fly tying materials.
