Changing yarn weight or needle size can have a significant impact on the finished project, so standardized systems have been spread about, as well as conversion systems for regional standards .
Yarn weight is important in achieving the correct gauge or tension for a particular project and can help with yarn substitution. The Craft Yarn Council of America has developed a system that seeks to standardize the labeled weights of yarn.Most yarns state their weight on the ball band but some may not, only giving the composition. Some brands use a standardized numbering system that uses seven ranges of relative thickness of yarn. However there are methods for individuals to gauge weight for themselves.
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.Comparing this with recommended needle sizes, yarn, and the knitter’s own signature tension, allows for adjustments to all of these things. For example, changing needle size is one way to bring the test swatch nearer to an accurate measurement in yarn weight.
The Gauge swatch goes further. Not only is it a tool for checking whether yarn conforms to a desired dimension, but it is usually produced with some of the complexities of the intended project (i.e. multiple colours, varied stitches, edgings) making it a much larger test piece. This larger sample is then “dressed” meaning washed, ironed, and subjected to other processes expected of the finished item. It is especially used for items that require a lot of work and time, to avoid dimensional mistakes in the long run.