Super S Number

Not a true unit of measure, S or super S number is an index of the fineness of the wool fiber and is most commonly seen as a label on wool apparel, wool fabric, and yarn.

An S number on the label of a wool suit, or other tailored wool apparel, indicates the fineness of the wool fiber used in the making of the apparel. The numbers may also be found on wool fabric and yarn. The S numbers for fine wool products are intended to state, with precision, the fineness of the wool fiber used in the product, as measured in micrometres.

The micrometre or micrometer, also commonly known as a micron, is an SI derived unit of length equaling 1×10−6 of a metre (SI standard prefix "micro-" = 10−6); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch).

fiber fineness is one of the factors determining the quality and performance of a wool product. In recent years it has also become an important marketing device used by many mills, garment makers, and retailers. The S number appears as a plural with an " s " or " 's " following the number, such as 100s or 100's.

The worsted count was only a rough measurement and has been replaced by more exact methods such as the metric yarn count. However, for fine wool, the memory of the older system survives in the S numbers which have a very rough correspondence to worsted count. In other words, fiber that yielded about 80 worsted hanks is roughly comparable to fiber designated as 80s using the S numbers. The critical difference is that while the worsted number was an indirect measure (and a rough one at that) of yarn thickness, the S number is a direct (and precise) measurement of the thickness of the fiber. The S number of fiber, and hence of yarn, fabric, or garments made of that fiber, is determined using the international standard promulgated by the International Wool Textile Organisation.

The International Wool Textile Organisation (IWTO) is the promulgator of the fabric Labelling Code of Practice which governs the use of the "S" and "Super S" designations for fine wool and wool blend fabrics. The Code defines the S number by correlation to maximum fiber diameter. For example, 80s must have maximum fiber diameter of 19.75 micrometres or finer and 90s, 19.25 micrometres or finer. This scale continues to the 210s at 13.25 micrometre or finer. Thus each step of ten (as from 80s to 90s or 90s to 100s) corresponds to 0.5 micrometre less in allowed maximum fiber diameter. It has been proposed to extend the scale to 250s at 11.25 micrometre or finer.

Additional reference for wool measurements:

A micron (micrometre) is the measurement used to express the diameter of wool fibre. Fine wool fibers have low micron value. Fibre diameter is the most important characteristic of wool in determining its greasy value.

Learn about suit fabrics on our fashion blog.

Learn more about wool measurements.

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Fineness Is a Trade-Off, Not a Trophy

Rising S numbers mean falling fiber diameters, and the cloth changes character as it climbs: higher-S fabrics feel smoother, drape more fluidly and weigh less, but the finest fibers are also more delicate, more prone to shine and wear, and less forgiving of daily use. A Super 100s cloth is often the working suit's sweet spot, while the highest S numbers belong to special-occasion garments worn in rotation rather than in combat. Honest tailors present the S number as one variable among several — weave, weight, finishing and construction matter as much — because a number meant to describe fiber fineness was never meant to grade a whole suit by itself.

From Fiber Diameter to Label Claim

The S scale began in the older spinning-count world, where finer wool yielded more yarn per pound, and modern definitions re-anchored the numbers to measured fiber diameter in micrometres, policed by wool industry bodies so that a label claim can be tested rather than merely asserted. That history explains the scale's odd feel: it behaves like a count, is regulated like a measurement, and is marketed like a grade. The wool pages linked from here — the wool measurement definition and the broader wool resources — give the surrounding vocabulary, and the suiting fabrics guide shows how S numbers sit alongside weight and weave when a suit cloth is actually chosen.