Textile & Yarn Converter

They evolved as two independent local industry conventions before any international standard existed. US sizing increases with needle diameter (bigger number = bigger needle), while UK/Canadian sizing was originally based on wire-gauge tables, which run the opposite way (bigger number = thinner wire). The metric system in millimeters sidesteps the problem by measuring the actual diameter directly.

WPI is the number of times a single strand of yarn wraps snugly around a ruler over one inch, without overlapping or stretching. Because it captures the real diameter of the spun yarn, it is the most reliable way to identify an unlabeled yarn or compare two yarns from different brands. Roughly: 14+ WPI is fingering, 12 is sport, 11 is DK, 9 is worsted, 7 is bulky.

Japanese needle sizing follows its own discrete steps (size 0 = 2.1 mm, size 15 = 6.6 mm) that do not align one-to-one with the US or metric system. The steps are smaller, so a Japanese pattern can specify a more precise needle than a US-only chart would offer. When converting, pick the nearest millimeter size unless the pattern specifies a particular gauge.

The Craft Yarn Council publishes a numbered system from 0 (Lace) to 7 (Jumbo) that most major US yarn brands use on their labels. The category groups several traditional names — for example, category 4 covers Worsted, Aran, and Afghan. Brands vary slightly within a category, so always knit a gauge swatch when substituting yarns.

Two knitters using the same yarn and the same labeled needle can produce different stitch counts per inch because of personal tension. Patterns are designed around a stitches-per-inch target, not the needle itself. If your gauge swatch is too tight, go up one needle size; if too loose, go down. The needle size on the label is a starting suggestion, not a rule.