Fly Tying Thread Sizes Explained: Choosing the Right Thread

The thread is one of the smallest things on a fly tying bench, but it affects almost every part of the finished fly. Choose one that is too bulky and a neat dry-fly head becomes difficult to finish. Use a thread that is too fine for the materials and tension required, and you may spend more time repairing broken wraps than tying.

If you've seen spools labelled 3/0, 6/0, 8/0, 12/0 or 140 denier and wondered which one to buy, the first thing to understand is that these labels do not form one universal sizing system. The best choice depends on the material used to make the thread, the maker's specification, the fly pattern and your tying technique.

For available colours and spool specifications, browse our fly tying threads.

What do 3/0, 6/0, 8/0 and 12/0 mean?

Traditional “aught” sizes give a broad indication of thread fineness. Within a comparable product range, higher numbers generally suggest a finer thread: 12/0 is intended for more delicate work than 3/0. Unfortunately, manufacturers have not always used identical measuring methods. An 8/0 thread from one maker can differ in diameter, strength and flatness from another 8/0.

Use the following as starting points, not fixed rules:

For example, Semperfli Classic Waxed Thread 8/0 is a waxed polyester option intended for controlled tying with relatively little bulk. Compare its manufacturer's details with any alternative, rather than relying on “8/0” alone.

What does denier measure?

Denier describes the mass, in grams, of 9,000 metres of fibre or yarn. A 70-denier yarn has half the mass per unit length of a 140-denier yarn, but that does not automatically make it half the diameter or half the strength. Differences in fibre, construction, twist and flattening matter.

Denier is particularly useful when comparing similar constructions from the same manufacturer. When switching between polyester, nylon, silk or GSP, treat it as one specification rather than a direct guide to how hard you can pull.

Waxed polyester, GSP and silk: what's the difference?

Waxed polyester is a versatile everyday choice. Its surface helps control materials and it suits a wide range of trout patterns. Many beginners find it easier to use than very slippery high-strength thread.

GSP (gel-spun polyethylene) offers impressive strength for its weight and is useful where you want to apply controlled tension without adding excessive bulk. It can be slippery, however, and excessive pressure can damage softer materials or cut into them. Wrap with care rather than assuming stronger thread always produces a better fly.

Natural silk can be desirable for traditional dressings and certain wet-fly patterns. Its handling and appearance are part of the appeal, but it needs to be chosen for the pattern rather than substituted blindly for a modern synthetic.

Should the thread be flat or twisted?

A flatter thread can spread pressure over a larger area and help keep bodies slim. A deliberately twisted thread behaves more like a cord and may help with some tying jobs, but it can build bulk quickly. Many threads change their behaviour as the bobbin rotates, so learning to control twist is often as important as choosing a different spool.

When building a narrow abdomen, use smooth wraps and check the body from the side before adding more material. When securing deer hair or bulky wings, prioritise a secure hold and the correct technique rather than simply using more thread turns.

Match the thread to the pattern

For a small dry fly, begin with a fine thread that lets you finish a neat head without crowding the eye. For a Pheasant Tail Nymph or Hare's Ear, an all-round thread that binds wire, dubbing and tail fibres cleanly is usually convenient. For a larger streamer, use a construction and strength suitable for securing longer materials or bulky heads.

If you are learning, buy one dependable general-purpose thread in two practical colours before collecting every size available. Black and a neutral olive or brown cover a surprising range of trout patterns. Add a fine thread or a specialised GSP spool when a specific pattern calls for it.

Common thread-size mistakes

The first is assuming the size label tells you everything. The second is using excessive tension because the thread appears strong. The third is piling on wraps to compensate for poor material placement. A well-positioned tail, wing or hackle usually needs fewer securing turns than a bundle that is already out of alignment.

Finally, check the pack and spool length when comparing prices. A lower ticket price does not always mean better value if the spool is substantially shorter.

Frequently asked questions

Is 8/0 a good all-round fly tying thread?

It can be an excellent general choice for many trout flies, particularly where a slim head matters. Check the specific manufacturer's thread characteristics and move up or down in size for your pattern and tying tension.

Is denier the same as breaking strength?

No. Denier measures linear mass, not the force required to break a thread. Material and construction influence the breaking strength.

Is GSP always better than polyester?

No. GSP is especially useful when strength-to-bulk matters, but waxed polyester can be easier to handle and gives excellent results for everyday patterns.

Can I use the same thread for dry flies and streamers?

Sometimes, but it depends on the hook size, materials and how much tension is needed. A general-purpose spool is a useful starting point, and a finer or stronger option can be added for specialised patterns.

Ready to choose a spool? Browse the full Fly Tying Threads category and compare the colour, manufacturer, size and spool length before ordering.