In this article
The short answer
The Super S number is a measure of one thing only: the maximum mean diameter of the wool fibre, in microns. Super 120s means 17.75 microns or finer. It says nothing about a fabric weight, weave, durability or country of origin. Under the IWTO code, the word "Super" is reserved for pure new wool - a wool blend can be labelled "120s" but never "Super 120s".
Almost every mid-price and premium suit in Britain carries a number on the swatch: Super 100s, Super 120s, sometimes Super 150s. Retailers present these as a quality ladder, with a higher number meaning a better suit. That is not what the number measures. The Super S system is a defined labelling standard with a published table behind it, and reading it properly tells you where the extra money goes - and, just as usefully, where it does not.
What the S number actually measures
The Super S system is set out in the International Wool Textile Organisation (IWTO) Fabric Labelling Code of Practice. It defines each S value by exactly one property: the maximum mean fibre diameter of the wool, measured in microns (one millionth of a metre). Nothing else.
The Super S system is set out in the International Wool Textile Organisation (IWTO) Fabric Labelling Code of Practice. It defines each S value by exactly one property: the maximum mean fibre diameter of the wool, measured in microns (one millionth of a metre). Nothing else.
The scale is arithmetic and completely predictable. Super 80s permits a mean fibre diameter of 19.75 microns or finer, and every ten-point step up the scale removes exactly 0.5 microns from the permitted maximum. Super 120s therefore means 17.75 microns or finer.
Two details matter here and are routinely lost in retail copy. First, this is a <em>mean</em> diameter across the fabric, not a guarantee about any individual fibre. Second, the measurement is taken at the finished product stage, using the IWTO-08 projection microscope method - so it describes the cloth you are buying, not the fleece it started as.
| S number | Maximum mean fibre diameter |
|---|---|
| Super 100s | 18.75 microns |
| Super 110s | 18.25 microns |
| Super 120s | 17.75 microns |
| Super 130s | 17.25 microns |
| Super 150s | 16.25 microns |
| Super 180s | 14.75 microns |
Sources: International Wool Textile Organisation (IWTO), The Woolmark Company
"Super 120s" and "120s" are not the same claim
This is the single most useful thing to know when reading a label, and it is the part retailers rarely explain.
This is the single most useful thing to know when reading a label, and it is the part retailers rarely explain.
Under the IWTO code, the word <strong>Super</strong> may only be used to describe fabric made from pure new wool. For a wool blend, the code is explicit: the word Super is not permitted. A blend may carry the bare S number - "120s" - provided the wool content is at least 45%.
So a fabric labelled <strong>120s</strong> rather than <strong>Super 120s</strong> may legitimately be under half wool. Both labels describe the same 17.75-micron fineness, but only one of them tells you what the rest of the cloth is. If a label shows a bare S number, the composition is the thing to check next.
The code does allow some exceptions within a "Super" fabric: blending with rare fibres such as mohair, cashmere and alpaca is permitted, as is silk, elastane for stretch, and up to 5% non-wool yarn for decorative effect. A "Super 130s" with 3% elastane is entirely legitimate.
Why finer is not simply better
It is worth putting these numbers next to the figure that governs whether wool actually feels comfortable. The Woolmark Company reports that sensations of itch and prickle occur when fibres coarser than roughly 30-32 microns press against the skin - and that this is mechanical irritation, not an allergic response.
It is worth putting these numbers next to the figure that governs whether wool actually feels comfortable. The Woolmark Company reports that sensations of itch and prickle occur when fibres coarser than roughly 30-32 microns press against the skin - and that this is mechanical irritation, not an allergic response.
Every fabric in the Super S table is far below that threshold. Super 100s, at 18.75 microns, is already well under it; so is Super 80s. This means the comfort argument for climbing the scale is much weaker than it is usually made to sound. You are not buying your way out of prickle at Super 150s, because you had already left it behind long before Super 100s.
What you are buying is handle and drape: finer fibre gives a softer, smoother, more fluid cloth with a cleaner line. That is a real and visible difference, and for an occasion suit it can be worth paying for.
There is a common trade tailoring convention on the other side of that decision: very high Super counts (150s and up) are generally sold and worn as occasion cloth, alongside genuinely rare fibres like cashmere and vicuna, rather than as daily-wear cloth. That is a convention, not a documented physical law - retailers and tailors treat these fabrics as more delicate to handle and more expensive to replace, which is reason enough to reserve them for suits worn occasionally rather than for a daily commute.
Sources: The Woolmark Company
What the number does not tell you
Because the S number is defined solely by fibre diameter, a long list of things that determine whether a suit is any good sit entirely outside it.
Because the S number is defined solely by fibre diameter, a long list of things that determine whether a suit is any good sit entirely outside it.
Fabric weight
A Super 120s can be woven at 7oz or at 13oz. Weight, not the S number, decides whether a suit works in July or in January.
Weave and finish
Twill, hopsack, birdseye and flannel all behave differently, and all can be made at the same fineness.
Construction
Whether a jacket is fully canvassed, half-canvassed or fused affects how it wears and ages far more than half a micron does.
Yarn and mill quality
The S number says nothing about spinning, dyeing or finishing - which is why two Super 120s fabrics can feel quite different.
Fit
A Super 150s suit that fits badly will always look worse than a Super 100s that fits properly.
What this means when you are actually buying
For a suit worn regularly - the office, client meetings, several weddings a year - the range from Super 100s to Super 130s is the practical sweet spot. It is comfortably finer than the prickle threshold, it drapes well, and it has not yet traded away the robustness that regular wear demands.
For a suit worn regularly - the office, client meetings, several weddings a year - the range from Super 100s to Super 130s is the practical sweet spot. It is comfortably finer than the prickle threshold, it drapes well, and it has not yet traded away the robustness that regular wear demands.
Reserve the very high numbers for suits worn occasionally. Above roughly Super 150s you are paying for a cloth that is measurably more delicate, in exchange for a refinement that is most visible when the suit is not being put through a commute.
And if a label shows a bare S number with no "Super", check the composition before anything else. That is the label doing exactly what the code permits, and it is telling you something.
Frequently Asked Questions
What does Super 120s wool mean?
Super 120s means the wool has a maximum mean fibre diameter of 17.75 microns or finer, as defined by the IWTO Fabric Labelling Code of Practice. The word "Super" additionally indicates the fabric is made from pure new wool. It is purely a measure of fibre fineness and says nothing about the fabric weight, weave or construction.
Is Super 150s better than Super 120s?
It is finer - 16.25 microns against 17.75 - but not automatically better. Finer wool drapes more softly and has a cleaner line, which is a real difference for an occasion suit. Very fine wool is also treated by convention as more delicate to handle and more expensive to replace, so for a suit worn regularly, Super 120s is generally the more practical choice; Super 150s suits occasional wear.
What is the difference between "120s" and "Super 120s"?
Both indicate a maximum mean fibre diameter of 17.75 microns, but under the IWTO code the word "Super" is reserved for pure new wool. A wool blend cannot be labelled "Super" and may be labelled "120s" only if it is at least 45% wool. A bare S number is therefore a prompt to check the fabric composition.
Does a higher Super number mean a more comfortable suit?
Not meaningfully. The Woolmark Company reports that prickle is caused by fibres coarser than about 30-32 microns, and every fabric on the Super S scale is far finer than that. Above Super 100s you are paying for handle and drape rather than for relief from prickle.
Is Super 120s wool durable enough for daily office wear?
Yes. Super 120s sits in the practical middle of the scale - fine enough to drape well, but well below the very high Super counts (150s and up) that are conventionally treated as occasional-wear cloth because they are more delicate to handle and more expensive to replace.
Can a suit be Super 120s and still be mostly polyester?
Not if it is labelled "Super". That word requires pure new wool, allowing only limited exceptions such as elastane for stretch or up to 5% decorative yarn. A bare "120s" label, however, only requires 45% wool - so the remainder could be synthetic.
How fine is 17.75 microns compared to human hair?
Considerably finer. The Woolmark Company puts the average human hair at 50 to 100 microns, so Super 120s wool is roughly three to five times finer than a typical hair.
Conclusion
The Super S number is a precise, well-defined and genuinely useful piece of information - it just answers a narrower question than most shoppers assume. It tells you how fine the wool is, and that is all it tells you. Read it alongside the weight, the composition and the construction, and it becomes a real tool. Read it as a quality score, and it will steer you towards a more delicate suit than you probably wanted.
References
- Fabric Labelling Code of Practice: "Super S" descriptions — International Wool Textile Organisation (IWTO)
- Woolmark Specification F-6: Wool Fibre Quality — The Woolmark Company, 2016
- Is wool comfortable for sensitive skin? — The Woolmark Company
- What is Merino wool and how is it made? — The Woolmark Company
Arthur Miller writes about suiting cloth for Search For Suits, working from published textile standards and mill specifications rather than retail marketing copy. He reads the IWTO and Woolmark specifications directly and checks retailer claims against them.