The Yarn Weight Chart below gives a handy comparison of different knitting and crochet yarns along with needle and hook sizes, approximate meterage to the 100g and WPI, using UK, US, German and Australian terminology.
Yarn weight is important in achieving the correct gauge or tension for a particular project and can help with yarn substitution. Most yarns state their weight on the ball band but some may not,this is particularly the case with Bulgarian and Turkish yarn manufacturers.
However by looking at the meterage,and recommended needle size in this yarn weight chart, which are also given on the ball band, it is possible to determine the weight of any given yarn.
Another useful method of determining weight is by, wraps per inch, or wpi. To do this wrap the yarn around a large needle or a ruler. Make sure that the yarn lies flat and push the yarn together so that there are no gaps between the wraps. Smooth it out so it is neither too loose nor too tight then measure the number of wraps per inch (2.5 cm). For better accuracy, measure the wraps at the center of your yarn sample.
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 DK,8 ply (worsted weight, medium weight) yarn.
Types of yarn weights:-