What Do Super Numbers Mean on Wool Cloth?

The Super number on a wool suit label, Super 100s, Super 120s, Super 150s and so on, is a certified measurement of fiber fineness. It comes from the International Wool Textile Organisation (IWTO), and every ten steps on the scale means the wool’s maximum fiber diameter is 0.5 micron finer than the step below it. So Super 100s tops out at 18.75 micron, Super 120s at 17.25 micron, and Super 150s at 16.00 micron. A higher number gives you a finer, softer cloth, and finer cloth also wears out faster, so the right number depends on how often you plan to wear the suit.

What a micron actually measures

A micron is a millionth of a meter, and in wool it measures the diameter of a single fiber, not its length or its quality in any broader sense. Labs test fineness with IWTO-approved methods: Airflow (IWTO-6), the Laserscan system that has become the default method for Merino certification (IWTO-12), OFDA (IWTO-47), and the older Projection Microscope method (IWTO-8). Testing labs have to be licensed by IWTO and are audited every year before their certificates carry any weight in the trade. That is why an IWTO certificate is treated as the gold standard for a fineness claim, and a number without that certification behind it is only as good as the word of whoever sold you the cloth.

For wool averaging 26.0 micron or finer, a properly performed IWTO test is accurate to within plus or minus 0.2 micron. That precision matters, because the entire Super scale is built on tenths of a micron.

The Super scale, step by step

The Super S system is owned and registered by IWTO and administered by The Woolmark Company. Under the IWTO Code of Practice, the word “Super” may only be used on fabric that is pure new wool. A blend can still quote a number, but it drops the word “Super” and is sold simply as, say, “120s”.

Where the number came from

Many trade sources repeat a story that the Super number once meant the number of hanks of 560-yard yarn that could be spun from one pound of wool, so a higher number meant a finer, more spinnable fiber. That explanation is useful for understanding why the scale exists, though sources disagree on how literally it should be taken today, and none of the places repeating it are primary IWTO documents. The certified number on a label today is defined purely by measured micron diameter under the Code of Practice. The spinning-count story explains where the scale came from historically, and today’s legal definition rests entirely on the micron test.

Why a higher number is not automatically better

Finer fiber drapes better, feels softer against the skin, and takes a sharper finish. It also abrades faster, shows shine at the elbows and seat sooner, and creases more readily than a coarser, sturdier fiber. Bespoke tailors and menswear writers generally point buyers of an everyday suit toward Super 100s to 130s, and keep Super 150s and above for suits that only come out on occasion. If you travel for work and live in a suit, a very high Super number is often the wrong choice even though it reads as the more luxurious label.

Two other properties of the fiber sit behind this trade-off. Wool has a natural wave called crimp, running roughly four to ten waves per centimeter, which gives the cloth its bounce and its ability to spring back into shape after being stretched; wool can extend up to around 30 to 35 percent of its length before it breaks. And raw wool carries lanolin, a natural wax that coats the fiber and is mostly washed out during processing. Neither of these changes with the Super number, but they are part of why wool behaves the way it does on the body, finer or not.

Fineness is only half the story

A Super number tells you about the diameter of the fiber, but it says nothing about how that fiber was prepared before spinning, and that step matters just as much to how the finished cloth behaves. Fine suiting wool is almost always worsted spun: the longer fibers are combed so they lie parallel to each other, and the short fibers and debris are stripped out along the way, which is what gives worsted cloth its smooth, dense, durable surface. Shorter fibers instead go to woollen processing, where carding leaves them disorganized, trapping more air for a fuzzier, warmer, but less durable yarn. A very fine Super number on poorly prepared fiber will still disappoint, so it is worth weighing the Super number alongside the weave and finish of the cloth too.

A word on “Merino micron”

You will sometimes see a single micron figure quoted for Merino wool as if it were one fixed number. It is not. Depending on the source and the specific bloodline, quoted ranges for commercial Merino run anywhere from about 13.5 to 32 micron, with a single fleece varying from around 10 to over 25 micron depending on the sheep’s age, health, and nutrition. Treat any single “Merino is X micron” claim with a little caution. The certified Super number printed on the finished cloth is a far more reliable guide to actual fineness than the breed name on its own.

Sources

1. International Wool Textile Organisation, “Wool Testing”, accessed 2026-09-25.
2. SGS, “Wool Testing Services Info Bulletin”, accessed 2026-09-25.
3. The Woolmark Company, “Product Specifications”, accessed 2026-09-25.
4. CCMI, “Definition of Cashmere, Fine and Superfine Wool”, accessed 2026-09-25.
5. Gentleman’s Gazette, “Wool Suit Fabric Super Numbers Explained”, accessed 2026-09-25.
6. The Natural Fibre Company, “The Wool Journey Part 3”, accessed 2026-09-25.
7. LanisLanolin, “Wool grease: Composition and specification”, accessed 2026-09-25.
8. The Woolmark Company, “What is Merino wool”, accessed 2026-09-25.
9. John Arbon Textiles, “Woollen vs Worsted Spinning”, accessed 2026-09-25.