 The number of wraps will act as a gauge to assess the thickness of unmarked yarn; for example 12 wraps per inch is 12 WPI, and is used to calculate how much yarn is required for various articles, so that 12 WPI is equivalent to 8 ply (worsted weight, medium weight) yarn.A more hands-on method, there is the Test swatch and the Gauge swatch.Knitting a Test swatch requires at its simplest, knitting the yarn into a small, roughly 4 in (10 cm) square textile of even stitches.