S number (wool)

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. 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 S number often is used in conjunction with the word "super" which originally meant the best wool, (called choice wool in the United States) As now used Super means pure new wool and can also be used for fabrics made from wool blended with rare fiber (such as mohair, cashmere wool and alpaca), and also with silk. The inclusion of Elastane to give the fabric a stretch effect is permitted, as also is the inclusion of up to 5% non-wool yarn for decorative effects. For wool blend fabric descriptions, the word Super is not permitted. Subject to the wool content being at least 45%, the S number may be used.

The S numbers originated in England, where the worsted spinning process was invented and arose from the worsted yarn count system for stating the fineness of yarn. The worsted count (also known as the Bradford count) was the number of 560-yard (510 m) lengths (hanks) of worsted yarn that 1 pound (0.45 kg) of wool yields. The finer the wool, the more yarn and the higher the count.

The worsted count was only a rough measurement and has been replaced by more exact methods such as the metric yarn count.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 (IWTO).

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